"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.
In 388, St. Augustine charged, in his
Morals of the Manichees
"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:
“[Q.] Certain married couples, relying on the opinion of learned physicians, are convinced that there are several days each month in which conception cannot occur. Are those who do not use the marriage right except on such days to be disturbed, especially if they have legitimate reasons for abstaining from the conjugal act?”
On March 2, 1853, the Sacred Penitentiary answered as follows:
“[A.] Those spoken of in the request are not to be disturbed, providing that they do nothing to impede conception.”
Obviously, this response is not meant to condemn artificial contraception yet endorse NFP, because artificial contraception is not needed when "conception cannot occur." The response simply indicates that it is not against nature to use marriage when conception cannot occur, but it is against nature to take measures to avoid/thwart/exclude/impede conception. This is articulated in Casti Connubii 59: "Nor are those considered as acting against nature who in the married state use their right in the proper manner although on account of natural reasons...new life cannot be brought forth. For in matrimony as well as in the use of the matrimonial rights there are also secondary ends, such as mutual aid, the cultivating of mutual love, and the quieting of concupiscence which husband and wife are not forbidden to consider so long as they are subordinated to the primary end.." ..the marriage act [] will always be evil unless it be excused by [offspring]." Aquinas/Summa q. 49 Art 5.
To "impede conception" is to subordinate the primary purpose of the act. So, the infertile window can be used as long as conception remains the primary purpose, not the purpose being avoided. The decree does not mention any specific type of impediment. It condemns all impediments, including deliberately-timed infertility.
The 1853 response says that no action may be taken by the spouses that would impede conception: The goal of NFP is to impede conception via timing when the spouses engage in their carefully-timed marital act. The purpose of NFP is "to avoid children and wish to make sure that none will result." Humanae Vitae 16.
The 1853 response said they can abstain from fertile relations if they have legitimate reason to do so, so long as that reason is not to avoid conception. This motive is not mentioned in the 1853 question and is even condemned in the last sentence, which says, “providing that they [spouses] do nothing to impede conception.” This is because "increase and multiply" is nature, not duty. There is obligation to produce children.
In 1880, NFP advocates tried again, this time with an overt purpose requested - to avoid "having numerous children."
Fr. Le Conte submitted the following questions to the Sacred Penitentiary in 1880:
“[Q.] Whether married couples may have intercourse during such sterile periods without committing mortal or venial sin?
“Whether the confessor may suggest such a procedure either to the wife who detests the onanism of her husband but cannot correct him, or to either spouse who shrinks from having numerous children?”
The response of the Sacred Penitentiary dated June 16, 1880, was:
“[A.] Married couples who use their marriage right in the aforesaid manner are not to be disturbed, and the confessor may suggest the opinion in question, cautiously, however, to those married people whom he has tried in vain by other means to dissuade from the detestable crime of onanism.”
1. There is no sin in having marital relations during known infertile periods, provided conception is not avoided deliberately.
2. The response does not permit NFP for spouses who shrink from having numerous children, but only for the sin of Onanism.
The second part of the response which supports NFP only allows it in case of Onanism: “...and the confessor may suggest the opinion in question, cautiously, however, to those married people whom he has tried in vain by other means to dissuade from the detestable crime of onanism.” The only permitted use of NFP, according to this response, would be if the husband obstinately commits the sin of Onanism. If not, the confessor cannot even suggest the use of NFP. This is articulated in Casti Connubii 59."Holy Church knows well that not infrequently one of the parties is sinned against rather than sinning, when for a grave cause he or she reluctantly allows the perversion of the right order (secondary over primary). In such a case, there is no sin, provided that, mindful of the law of charity, he or she does not neglect to seek to dissuade and to deter the partner from sin." #59 is not referring to artificial contraception as decreed by the Sacred Penitentiary in 1916.
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).
(47) 3 Cf. Denzinger-Schönmetzer, Enchiridion Symbolorum, 2795, 3634.
NFP/Onanism is not "already illicit" like artificial contraception is, so it can be tolerated initially, but rejected as sin finally. Footnote 47 is to the Sacred Penitentiary rulings above. There can be no cooperation with the sin of artificial contraception. There can be cooperation with the sin of NFP.
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).
