This should really be a separate thread. I think the real issue is the idea of planning the size of your family, as the actual family size is purely accidental, if "rhythm" is used properly for some grave reason, as the Church teaches.

Birth control

Today, besides, another grave problem has arisen, namely, if and how far the obligation of being ready for the service of maternity is reconcilable with the ever more general recourse to the periods of natural sterility the so-called "agenesic" periods in woman, which seems a clear expression of a will contrary to that precept.

You are expected to be well informed, from the medical point of view, in regard to this new theory and the progress which may still be made on this subject, and it is also expected that your advice and assistance shall not be based upon mere popular publications, but upon objective science and on the authoritative judgment of conscientious specialists in medicine and biology. It is your function, not the priest's, to instruct the married couple through private consultation or serious publications on the biological and technical aspect of the theory, without however allowing yourselves to be drawn into an unjust and unbecoming propaganda. But in this field also your apostolate demands of you, as women and as Christians, that you know and defend the moral law, to which the application of the theory is subordinated. In this the Church is competent.
It is necessary first of all to consider two hypotheses. If the application of that theory implies that husband and wife may use their matrimonial right even during the days of natural sterility no objection can be made. In this case they do not hinder or jeopardize in any way the consummation of the natural act and its ulterior natural consequences. It is exactly in this that the application of the theory, of which We are speaking, differs essentially from the abuse already mentioned, which consists in the perversion of the act itself. If, instead, husband and wife go further, that is, limiting the conjugal act exclusively to those periods, then their conduct must be examined more closely.

Here again we are faced with two hypotheses. If, one of the parties contracted marriage with the intention of limiting the matrimonial right itself to the periods of sterility, and not only its use, in such a manner that during the other days the other party would not even have the right to ask for the debt, than this would imply an essential defect in the marriage consent, which would result in the marriage being invalid, because the right deriving from the marriage contract is a permanent, uninterrupted and continuous right of husband and wife with respect to each other.

However if the limitation of the act to the periods of natural sterility does not refer to the right itself but only to the use of the right, the validity of the marriage does not come up for discussion. Nonetheless, the moral lawfulness of such conduct of husband and wife should be affirmed or denied according as their intention to observe constantly those periods is or is not based on sufficiently morally sure motives. The mere fact that husband and wife do not offend the nature of the act and are even ready to accept and bring up the child, who, notwithstanding their precautions, might be born, would not be itself sufficient to guarantee the rectitude of their intention and the unobjectionable morality of their motives.

The reason is that marriage obliges the partners to a state of life, which even as it confers certain rights so it also imposes the accomplishment of a positive work concerning the state itself. In such a case, the general principle may be applied that a positive action may be omitted if grave motives, independent of the good will of those who are obliged to perform it, show that its performance is inopportune, or prove that it may not be claimed with equal right by the petitioner—in this case, mankind.

The matrimonial contract, which confers on the married couple the right to satisfy the inclination of nature, constitutes them in a state of life, namely, the matrimonial state. Now, on married couples, who make use of the specific act of their state, nature and the Creator impose the function of providing for the preservation of mankind. This is the characteristic service which gives rise to the peculiar value of their state, the bonum prolis. The individual and society, the people and the State, the Church itself, depend for their existence, in the order established by God, on fruitful marriages. Therefore, to embrace the matrimonial state, to use continually the faculty proper to such a state and lawful only therein, and, at the same time, to avoid its primary duty without a grave reason, would be a sin against the very nature of married life.
Serious motives, such as those which not rarely arise from medical, eugenic, economic and social so-called "indications," may exempt husband and wife from the obligatory, positive debt for a long period or even for the entire period of matrimonial life. From this it follows that the observance of the natural sterile periods may be lawful, from the moral viewpoint: and it is lawful in the conditions mentioned. If, however, according to a reasonable and equitable judgment, there are no such grave reasons either personal or deriving from exterior circuмstances, the will to avoid the fecundity of their union, while continuing to satisfy to tile full their sensuality, can only be the result of a false appreciation of life and of motives foreign to sound ethical principles.

The heroism of continence

Perhaps you will now press the point, however, observing that in the exercise of your profession you find yourselves sometimes faced with delicate cases, in which, that is, there cannot be a demand that the risk of maternity be run, a risk which in certain cases must be absolutely avoided, and in which as well the observance of the agenesic periods either does not give sufficient security, or must be rejected for other reasons. Now, you ask, how can one still speak of an apostolate in the service of maternity?

If, in your sure and experienced judgment, the circuмstances require an absolute "no," that is to say, the exclusion of motherhood, it would be a mistake and a wrong to impose or advise a "yes." Here it is a question of basic facts and therefore not a theological but a medical question; and thus it is in your competence. However, in such cases, the married couple does not desire a medical answer, of necessity a negative one, but seeks an approval of a "technique" of conjugal activity which will not give rise to maternity. And so you are again called to exercise your apostolate inasmuch as you leave no doubt whatsoever that even in these extreme cases every preventive practice and every direct attack upon the life and the development of the seed is, in conscience, forbidden and excluded, and that there is only one way open, namely, to abstain from every complete performance of the natural faculty. Your apostolate in this matter requires that you have a clear and certain judgment and a calm firmness.

It will be objected that such an abstention is impossible, that such a heroism is asking too much. You will hear this objection raised; you will read it everywhere. Even those who should be in a position to judge very differently, either by reason of their duties or qualifications, are ever ready to bring forward the following argument: "No one is obliged to do what is impossible, and it may be presumed that no reasonable legislator can will his law to oblige to the point of impossibility. But for husbands and wives long periods of abstention are impossible. Therefore they are not obliged to abstain; divine law cannot have this meaning."

In such a manner, from partially true premises, one arrives at a false conclusion. To convince oneself of this it suffices to invert the terms of the argument: "God does not oblige anyone to do what is impossible. But God obliges husband and wife to abstinence if their union cannot be completed according to the laws of nature. Therefore in this case abstinence is possible." To confirm this argument, there can be brought forward the doctrine of the Council of Trent, which, in the chapter on the observance necessary and possible of referring to a passage of St. Augustine, teaches: "God does not command the impossible but while He commands, He warns you to do what you can and to ask for the grace for what you cannot do and He helps you so that you may be able".

Do not be disturbed, therefore, in the practice of your profession and apostolate, by this great talk of impossibility. Do not be disturbed in your internal judgment nor in your external conduct. Never lend yourselves to anything which is contrary to the law of God and to your Christian conscience! It would be a wrong towards men and women of our age to judge them incapable of continuous heroism. Nowadays, for many a reason,—perhaps constrained by dire necessity or even at times oppressed by injustice—heroism is exercised to a degree and to an extent that in the past would have been thought impossible. Why, then, if circuмstances truly demand it, should this heroism stop at the limits prescribed by the passions and the inclinations of nature? It is clear: he who does not want to master himself is not able to do so, and he who wishes to master himself relying only upon his own powers, without sincerely and perseveringly seeking divine help, will be miserably deceived.

Here is what concerns your apostolate for winning married people over to a service of motherhood, not in the sense of an utter servitude under the promptings of nature, but to the exercise of the rights and duties of married life, governed by the principles of reason and faith.

The final aspect of your apostolate concerns the defense of both the right order of values and of the dignity of the human being.

The order of values

"Personal values" and the need to respect such are a theme which, over the last twenty years or so, has been considered more and more by writers. In many of their works, even the specifically sɛҳuąƖ act has its place assigned, that of serving the "person" of the married couple. The proper and most profound sense of the exercise of conjugal rights would consist in this, that the union of bodies is the expression and the realization of personal and affective union.
Articles, chapters, entire books, conferences, especially dealing with the "technique" of love, are composed to spread these ideas, to illustrate them with advice to the newly married as a guide in matrimony, in order that they may not neglect, through stupidity or a false sense of shame or unfounded scruples, that which God, Who also created natural inclinations, offers them. If from their complete reciprocal gift of husband and wife there results a new life, it is a result which remains outside, or, at the most, on the border of "personal values"; a result which is not denied, but neither is it desired as the center of marital relations.

According to these theories, your dedication for the welfare of the still hidden life in the womb of the mother, anti your assisting its happy birth, would only have but a minor and secondary importance.

Now, if this relative evaluation were merely to place the emphasis on the personal values of husband and wife rather than on that of the offspring, it would be possible, strictly speaking, to put such a problem aside. But, however, it is a matter of a grave inversion of the order of values and of the ends imposed by the Creator Himself. We find Ourselves faced with the propagation of a number of ideas and sentiments directly opposed to the clarity, profundity, and seriousness of Christian thought. Here, once again, the need for your apostolate. It may happen that you receive the confidences of the mother and wife and are questioned on the more secret desires and intimacies of married life. How, then, will you be able, aware of your mission, to give weight to truth and right order in the appreciation and action of the married couple, if you yourselves are not furnished with the strength of character needed to uphold what you know to be true and just?

The primary end of marriage

Now, the truth is that matrimony, as an institution of nature, in virtue of the Creator's will, has not as a primary and intimate end the personal perfection of the married couple but the procreation and upbringing of a new life. The other ends, inasmuch as they are intended by nature, are not equally primary, much less superior to the primary end, but are essentially subordinated to it. This is true of every marriage, even if no offspring result, just as of every eye it can be said that it is destined and formed to see, even if, in abnormal cases arising from special internal or external conditions, it will never be possible to achieve visual perception.

It was precisely to end the uncertainties and deviations which threatened to diffuse errors regarding the scale of values of the purposes of matrimony and of their reciprocal relations, that a few years ago (March 10, 1944), We Ourselves drew up a declaration on the order of those ends, pointing out what the very internal structure of the natural disposition reveals. We showed what has been handed down by Christian tradition, what the Supreme Pontiffs have repeatedly taught, and what was then in due measure promulgated by the Code of Canon Law. Not long afterwards, to correct opposing opinions, the Holy See, by a public decree, proclaimed that it could not admit the opinion of some recent authors who denied that the primary end of marriage is the procreation and education of the offspring, or teach that the secondary ends are not essentially subordinated to the primary end, but are on an equal footing and independent of it.

Would this lead, perhaps, to Our denying or diminishing what is good and just in personal values resulting from matrimony and its realization? Certainly not, because the Creator has designed that for the procreation of a new life human beings made of flesh and blood, gifted with soul and heart, shall be called upon as men and not as animals deprived of reason to be the authors of their posterity. It is for this end that the Lord desires the union of husband and wife. Indeed, the Holy Scripture says of God that He created man to His image and He created him male and female, and willed—as is repeatedly affirmed in Holy Writ—that "a man shall leave mother and father, and shall cleave to his wife: and they shall be two in one flesh".

All this is therefore true and desired by God. But, on the other hand, it must not be divorced completely from the primary function of matrimony—the procreation of offspring. Not only the common work of external life, but even all personal enrichment—spiritual and intellectual—all that in married love as such is most spiritual and profound, has been placed by the will of the Creator and of nature at the service of posterity. The perfect married life, of its very nature, also signifies the total devotion of parents to the well-being of their children, and married love in its power and tenderness is itself a condition of the sincerest care of the offspring and the guarantee of its realization.

To reduce the common life of husband and wife and the conjugal act to a mere organic function for the transmission of seed would be but to convert the domestic hearth, the family sanctuary, into a biological laboratory. Therefore, in Our allocution of September 29, 1949, to the International Congress of Catholic Doctors, We expressly excluded artificial insemination in marriage. The conjugal act, in its natural structure, is a personal action, a simultaneous and immediate cooperation of husband and wife, which by the very nature of the agents and the propriety of the act, is the expression of the reciprocal gift, which, according to Holy Writ, effects the union "in one flesh".

That is much more than the union of two genes, which can be effected even by artificial means, that is, without the natural action of husband and wife. The conjugal act, ordained and desired by nature, is a personal cooperation, to which husband and wife, when contracting marriage, exchange the right.

Therefore, when this act in its natural form is from the beginning perpetually impossible, the object of the matrimonial contract is essentially vitiated. This is what we said on that occasion: "Let it not be forgotten: only the procreation of a new life according to the will and the design of the Creator carries with it in a stupendous degree of perfection the intended ends. It is at the same time in conformity with the spiritual and bodily nature and the dignity of the married couple, in conformity with the happy and normal development of the child".
Advise the fiancée or the young married woman who comes to seek your advice about the values of matrimonial life that these personal values, both in the sphere of the body and the senses and in the sphere of the spirit, are truly genuine, but that the Creator has placed them not in the first, but in the second degree of the scale of values.

Just the words, not the teaching.  Since what the bishop teaches is in essence what even Dr. Droleskey says is the Catholic Church's teaching regarding NFP, to say he "does not understand the Church’s moral teachings on natural family planning" because of Dr. Droleskey's opinion on mere semantics is deceptive in my book.

John, precise speech does matter.

But precise speech without charity is tinkling brass and so forth.

Quote

51. This council realizes that certain modern conditions often keep couples from arranging their married lives harmoniously, and that they find themselves in circuмstances where at least temporarily the size of their families should not be increased. As a result, the faithful exercise of love and the full intimacy of their lives is hard to maintain. But where the intimacy of married life is broken off, its faithfulness can sometimes be imperiled and its quality of fruitfulness ruined, for then the upbringing of the children and the courage to accept new ones are both endangered.

To these problems there are those who presume to offer dishonorable solutions indeed; they do not recoil even from the taking of life. But the Church issues the reminder that a true contradiction cannot exist between the divine laws pertaining to the transmission of life and those pertaining to authentic conjugal love.

For God, the Lord of life, has conferred on men the surpassing ministry of safeguarding life in a manner which is worthy of man. Therefore from the moment of its conception life must be guarded with the greatest care while abortion and infanticide are unspeakable crimes. The sɛҳuąƖ characteristics of man and the human faculty of reproduction wonderfully exceed the dispositions of lower forms of life. Hence the acts themselves which are proper to conjugal love and which are exercised in accord with genuine human dignity must be honored with great reverence. Hence when there is question of harmonizing conjugal love with the responsible transmission of life, the moral aspects of any procedure does not depend solely on sincere intentions or on an evaluation of motives, but must be determined by objective standards. These, based on the nature of the human person and his acts, preserve the full sense of mutual self-giving and human procreation in the context of true love. Such a goal cannot be achieved unless the virtue of conjugal chastity is sincerely practiced. Relying on these principles, sons of the Church may not undertake methods of birth control which are found blameworthy by the teaching authority of the Church in its unfolding of the divine law.(14)
All should be persuaded that human life and the task of transmitting it are not realities bound up with this world alone. Hence they cannot be measured or perceived only in terms of it, but always have a bearing on the eternal destiny of men.

In section 51 of Gaudium et Spes is the following footnote:

14. Cf. Pius XI, encyclical letter Casti Connubii: AAS 22 (1930): Denz-Schoen. 3716-3718; Pius XII, Allocutio Conventui Unionis Italicae inter Obstetrices, Oct. 29, 1951: AAS 43 (1951), PP. 835-854, Paul VI, address to a group of cardinals, June 23 1964: AAS 56 (1964), PP. 581-589. Certain questions which need further and more careful investigation have been handed over, at the command of the Supreme Pontiff, to a commission for the study of population, family, and births, in order that, after it fulfills its function, the Supreme Pontiff may pass judgment. With the doctrine of the magisterium in this state, this holy synod does not intend to propose immediately concrete solutions.

Ambrose, the Church never used the term "rhythm method" either.  Various churchmen did, but there is no clear teaching from a Pope that we are to call it the rhythm method.  There is no official name for it.

In the Allocution to Midwives, where Pius XII first teaches it, he never uses the term rhythm method.  He talks about the use of the "agenesic periods" ( sterile periods ) and also talks about "observance of the natural sterile periods."  Never does he say rhythm method.  

So if you want to be strictly accurate, don't use either term, whether rhythm method or NFP, but call this use of marriage the "observance of the natural sterile periods."