"NFP" is as old as the hills, and has always been condemned by the Catholic Church because the essence of marriage has procreation alone as it's primary natural purpose.
In the Old Testament, God warned husbands to consider their wives “unclean” (and therefore legally unapproachable) for the duration of her menstrual period and for the following seven days (Leviticus 15:28). The twelfth day, as we now know, is often the day of ovulation ... In other words, the Jewish law is a prescription for optimum fertility for most couples.
"Is it not you who used to warn us to watch, as much as we could, the time after purification of the menses, when a woman is likely to conceive, and at that time refrain from intercourse?" This eminent “Doctor of Grace” further argued against these heretics that “there is no marriage where motherhood is not in view; consequently, neither is there a wife”; therefore, he declared that such a union “makes the woman not a wife, but a mistress.” Pius XI repeats St. Augustine's words on December 31, 1930 in his encyclical Casti Connubii #65.
Meanwhile, because of the advancement of more precise fertility knowledge, in 1853, the Bishop of Amiens, France, submitted the following question to the Sacred Apostolic Penitentiary:
VADEMECUM FOR THE USE OF CONFESSORS 1997
13. Special difficulties are presented by cases of cooperation in the sin of a spouse who voluntarily renders the unitive [conjugal] act infecund. In the first place, it is necessary to distinguish cooperation in the proper sense, from violence or unjust imposition on the part of one of the spouses, which the other spouse in fact cannot resist.46, 561).] This cooperation can be licit when the three following conditions are jointly met:
when the action of the cooperating spouse is not already illicit in itself;47
when proportionally grave reasons exist for cooperating in the sin of the other spouse;
when one is seeking to help the other spouse to desist from such conduct (patiently, with prayer, charity and dialogue; although not necessarily in that moment, nor on every single occasion).
Casti Connubii 53-54 "Others say that they cannot on the one hand remain continent nor on the other can they have children because of the difficulties whether on the part of the mother or on the part of family circumstances. But no reason, however grave, may be put forward by which anything intrinsically against nature may become conformable to nature and morally good." Pius XII adds "Spouses, who make use of their matrimonial rights, have the positive obligation, by virtue of the natural law of their state, not to exclude procreation." 1958 International society of Hematology. "
Conclusion: the responses of the Sacred Penitentiary allow for the infertile only window when the reason thereof is circumstantial, and not purposed to avoid pregnancy. If the couple is avoiding pregnancy through complete continence, yet one spouse pursues Onanism, then said [NFP] window can be "reluctantly allowed" by the innocent spouse. However, said innocent spouse must "not neglect to seek to dissuade and to deter the partner from sin" (of the perversion of the right order -- seeking the secondary end of marriage independent of the primary). St. Alphonsus Liguori: Counseling the Penitent: "It is lawful to persuade to a lesser evil, if otherwise he is already determined to execute a greater. The reason is that then the one persuading does not seek evil, but good, namely, the choice of the lesser evil". This is why the sin of Onan is improved by the lesser sin of NFP. To quote St Francis de sales: sometimes it kills it effectually by mortal sin, as when the order appointed for the procreation of children is violated and perverted; in which case according as one departs more or less from it, the sins are more or less abominable, but always mortal :