"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."
Pius is saying that he is concerned that the increased popularity of using the understanding of women's fertile and infertile periods is indicative of a contraceptive mentality. It seems to him that general recourse to such knowledge is contrary to a woman's obligation to be ready for maternity. It should be noted that Pius mentioned only women not becausemen's obligation to be ready for paternity isany less binding, but rather because his audience was made up of midwives, who aid women in childbirth. In the following, he then attempts to explicate which scenarios involve licit or illicit use of increasing knowledge of fertility.
"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."
In other words, if you're using NFP to merely monitor the fertility and health of the wife, but not restricting your engagement in the marital act, there's no doubt that you're acting morally. If, however, you are limiting your sexual activity to just the infertile periods, further examination is required to determine the licitness of your behavior.
"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."
There's nothing new here. If you enter marriage with no intention whatsoever of conceiving children (assuming you are able), by only having sex when the wife (whoever's choice that may be) is infertile for instance, you lied when you took your wedding vows. That's sufficient ground for annulment. That is, your hostility to parenthood is an impediment to the valid operation of the Sacrament of holy Matrimony.
"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."
That is, if you're not denying the right of marital union to your spouse, the validity of the sacrament is not in question. However, the morality of the particular use of sex may still be in doubt. I think it's crucial to point out here that readiness to accept and raise a child conceived "accidentally" is insufficient to guarantee that one's use of periodic abstinence is moral. In other words, it's not enough to begrudgingly accept unplanned children. Presumably, he means that children should be joyfully, not begrudgingly, accepted. The reasons for putting off parenthood must be "sufficiently morally sure". What that means is unclear thus far.
"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."
Worth noting is His Holiness' use of "omitted". This seems to be in agreement with my contention (made in the preface to this series) that the commission of an immoral act is not the same as the omission of a moral act. Beyond that, it might seem that the previous paragraph, that motives for avoiding conception must be "grave",is just repeated with greater emphasis. Actually, Pius is making a much stronger statement.He says that married couples have a God-given natural duty to be fruitful. In fact, it's a couple's primary duty. The phrase "bonum prolis" means "the good of progeny", that is, children add to the goodness of marriage. To avoid that "primary duty without a grave reason, would be a sin against the very nature of married life."
It's not enough to talk only about reasons against children. If one's discernment in any situation is to be genuine, one must first consider the reasons for a given decision first. In this case, when Pius says that children are "a positive work concerning the state [of marriage] itself", that is,they a complement to the sacrament of matrimony. They are part of the the natural, cosmicorder of the universe and of man's personal and communal society, without which there is defect, i.e. disorder. Taking a peek a head to John Paul II, we can note that he regarded the "culture of death" as a manifestation of that disorder. Put simply, the conceiving and raising of children is not merely a private matter for a couple to consider, but a decision that affects the whole natural and social order. If we are to properly discern when avoiding conception is licit, we must bear that in mind.