Saint Caesar of Arles: “As often as he knows his wife without a desire for children… without a doubt he commits sin.”(7)
Errors Condemned by Pope Innocent XI: “9. The act of marriage exercised for pleasure only is entirely free of all fault and venial defect.”–Condemned (8)
Tobias 6:17 – “The holy youth Tobias approaches his bride Sara after three days of prayer, not for fleshly lust but only for the love of posterity. Having been instructed by the Archangel Saint Rapheal that to engage in the marital act he must be moved rather for love of children than for lust. For they who in such manner receive matrimony, as to shut out God from themselves, and from their mind, and to give themselves to their lust, as the horse and mule, which have not understanding, over them the devil hath power.”
“But no reason, however grave, may be put forward by which anything intrinsically against nature may become conformable to nature and morally good. Since, therefore, the conjugal act is destined primarily by nature for the begetting of children, those who in exercising it deliberately frustrate its natural powers and purpose sin against nature and commit a deed which is shameful and intrinsically vicious.
“Small wonder, therefore, if Holy Writ bears witness that the Divine Majesty regards with greatest detestation this horrible crime and at times has punished it with death. As St. Augustine notes, ‘Intercourse even with one’s legitimate wife is unlawful and wicked where the conception of offspring is prevented.’ Onan, the son of Judah, did this and the Lord killed him for it (Gen. 38:8-10).
~ Pope Pius XI: Casti Connubii
Those lukewarm "Catholics" who haven't even considered that NFP might be sinful and researched the issue will be repaid for all the pleasures they partook in this life unlawfully.
https://schismatic-home-aloner.com/natural-family-planning-nfp/