From Tom:
Far from "teaching" a natural method upon married couples to avoid the conception of children whenever they believed that it was opportune for them to do so, Pope Pius XII explained that there were merely certain extenuating circuмstances that might make it permissible to do so. This is far, far different than what is called today "natural family planning," replete with an immersion in graphic terms that would never pass from the lips of one concerned about modesty of speech or even be discussed openly by a husband and wife who understand the true nature of marital continence absent serious conditions that would be addressed in the confessional and/or in spiritual counseling, upon all couples. Please judge for yourselves:
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. (Pope Pius XII, Address to Midwives on the Nature of Their Profession, October 29, 1951.)
This is a specific rejection of the liberal view provided in a few of the ethics books used in seminaries in the 1940s and 1950s. It is not enough to simply be "open" to the birth of a child without using any artificial means to interfere with conception. Pope Pius XII taught that this "would not be itself sufficient to guarantee the rectitude of their intention and the unobjectionable morality of their motives."
Pope Pius XII went on to explain the reason that this is so:
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 to the full their sensuality, can only be the result of a false appreciation of life and of motives foreign to sound ethical principles. (Pope Pius XII, Address to Midwives on the Nature of Their Profession, October 29, 1951.)
It stands reason and truth on their heads to contend that the Address to Midwives on the Nature of Their Profession was designed to instruct engaged and recently married couples in how to use a woman's monthly periods of sterility to avoid conception as a matter of routine without the presence of the grave reasons listed by Pope Pius XII. No such "mandate" exists. None.