Pius XII says in his address to midwives:
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.
The first underlined part, is saying, that you can't restrict people from the marital act on the days the woman is infertile, HOWEVER, the second part states that something is wrong when people restrict their times of intercourse to ONLY the time where they're infertile.
He continues:
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.
This is saying to the mother that says, as her husband asks to consumate: "Hey honey, I'm fertile right now, can we not do it right now?"
Then he continues again:
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.
He's saying here, there has to be a MORALLY SURE reason (like the ones my confessor gave.)
And he continues again:
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.