Here's why. The only essential purpose of the conjugal act is procreation. Obviously during pregnancy, the essential purpose cannot be considered. "both in marriage itself and in the use of matrimonial law, secondary ends are also contained, such as mutual help and mutual affection to be fomented and the quiet of concupiscence, ends which the spouses are not prohibited from wanting, provided that it is the intrinsic nature of the act is always respected and, consequently, its subordination to the main purpose." Casti Connubii (Latin translation). Therefore, nature is offended by relations during pregnancy. it is unnatural and unreasonable to sow one’s seed when one “awaits the harvest.”
Athenagoras the Athenian “After throwing the seed into the ground, the farmer awaits the harvest. He does not sow more seed on top of it. Likewise, to us the procreation of children is the limit of our indulgence in appetite.”
St. Clement of Alexandria “To…a spiritual man, after conception, his wife is as a sister and is treated as if of the same father.”
St. Augustine, in his book On The Good of Marriage (A.D. 401), likewise agreed with the Church’s tradition that performing the marital act during pregnancy is unreasonable and unnatural since “necessary sexual intercourse for begetting [of children] is free from blame, and itself is alone worthy of marriage. But that which goes beyond this necessity [of begetting children] no longer follows reason but lust…” (Section 11) He also taught that marital relations during pregnancy “are the sins of the married persons themselves, not the fault of marriage.”
"the desire for union does not prevail, but ceases when there is no prospect of generation.” (St. Augustine, Against Julian, Book III, Chapter 21:43) Thus the conception of children is “the one alone worthy fruit… of the sexual intercourse.” (St. Augustine, On the Good of Marriage, Section 1)
St. Ambrose: "For even the young who temper their hearts to prudence by divine fear, generally renounce the works of youth when progeny [offspring] have been received. And is this remarkable for man, if beasts mutely speak a zeal for generating, not a desire for copulating? Indeed, once they know the womb is filled, and the seed received by the generative soil, they no longer indulge in intercourse or the wantonness of love, but they take up parental care. Yet men spare neither the embryo nor God. They contaminate the former and exasperate the latter. "
"even in the elements and the beasts it is a shame to nature not to cease from generating.” - St. Ambrose
"even these acts themselves are performed to the glory of God if they are attended to with a view to posterity [offspring] alone.” - Origen
"..only for the love of posterity" (Tobias 8:9)
"I would like to know, dearly beloved, what kind of a harvest a man could gather if he sowed his field in one year as often as he is overcome by dissipation and abuses his wife without any desire for children. If those who are unwilling to control themselves plowed and sowed repeatedly their land which was already sown, let us see in what kind of fruit they would rejoice. As you well know, no land can produce proper fruit if it is sown frequently in one year. Why, then, does a man do with his body what he does not want done with his field?” St. Caesarius of Arles
If NFP-ers seek self-control and discipline, look no further.
"Married people, then, must mutually abstain...after the wife has conceived he shall not have intercourse with her until she has borne her child, and they shall come together again for this purpose, as saith the Apostle. But if they shall fulfill this instruction, then they are worthy of the body of Christ… and there they shall receive the thirty-fold fruit which as the Savior relates in the Gospel, he has also plucked for married people." St. Finnian of Clonard
"It is lawful for you to take sensual pleasures only from your wife in order to beget legitimate offspring, for only these pleasures are lawful according to the Word.." St Clement of Alexandria
"continence after conception preserves the fruit of the womb from many sinful impulses" Anne Catherine Emmerich
“Men shall be… lovers of pleasure more than of God.” (2 Timothy 3:1-5)
"But the act of fornication is always evil. Therefore the marriage act also will always be evil unless it be excused by the aforesaid goods [begetting children, or of paying the debt]. - Aquinas
St. Bridget was allowed to communicate with this tormented soul. She asked the man about the specific reasons why he escaped Eternal Hell. He answered saying: “The third [reason I escaped being eternally condemned to burn in Hell] is that I obeyed my teacher who advised me to abstain from my wife’s bed when I understood that she was pregnant.” (The Revelations of St. Bridget
"[Our Lord] rigorously commanded perfect continence after the period of conception." Anne Catherine Emmerich
"the husband has no right to ask for the debt during this period" because marriage contract rights are only for "acts suitable for the generation of children."
“Those who have intercourse with the pregnant are murderers. I do not want the work of a man and woman to take place from the time when the root of a little child has already been placed in a woman, lest the development of that little child be polluted by excessive and wasted semen” St. Hildegard, doctor of the Church
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 [NFP/ "during pregnancy"]. 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. 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 either of time or of certain defects, new life cannot be brought forth." "During pregnancy" is acting against nature because it is neither defect, nor "time" (menopause). The right order is procreation purposed first, and secondary ends dependent thereon.
This blogpost is primarily my summation from a much longer article https://docs.google.com/document/d/1c6F6ac7tPNQgBetg0Aojyjz1833JzaB2krzvGvWASj0/edit?usp=sharing
