Why hasn’t anyone actually posted the “very strong comment” from St. Augustine?
65. Lastly, there is the symbol of the breast, in which your very questionable chastity consists. For though you do not forbid sexual intercourse, you, as the apostle long ago said, forbid marriage in the proper sense, although this is the only good excuse for such intercourse. No doubt you will exclaim against this, and will make it a reproach against us that you highly esteem and approve perfect chastity, but do not forbid marriage, because your followers—that is, those in the second grade among you—are allowed to have wives. After you have said this with great noise and heat, I will quietly ask, Is it not you who hold that begetting children, by which souls are confined in flesh, is a greater sin than cohabitation? Is it not you who used to counsel us to observe as much as possible the time when a woman, after her purification, is most likely to conceive, and to abstain from cohabitation at that time, lest the soul should be entangled in flesh? This proves that you approve of having a wife, not for the procreation of children, but for the gratification of passion. In marriage, as the marriage law declares, the man and woman come together for the procreation of children. Therefore whoever makes the procreation of children a greater sin than copulation, forbids marriage, and makes the woman not a wife, but a mistress, who for some gifts presented to her is joined to the man to gratify his passion. Where there is a wife there must be marriage. But there is no marriage where motherhood is not in view; therefore neither is there a wife. In this way you forbid marriage. Nor can you defend yourselves successfully from this charge, long ago brought against you prophetically by the Holy Spirit.
It isn’t really a strong condemnation of NFP at all, but a condemnation of a very specific group of people who taught that it was a sin to create a child and and thus trap a soul into a physical form. They had removed the pro-creative aspect of sex entirely. This certainly doesn’t count as clear evidence, and there needs to be an additional source besides Augustine’s teachings because we don’t follow every single thing he said anyway.
Augustine’s line of reasoning is very evident. He, at the same time, attacks heresy by exposing it’s teachings (that procreation is evil, because it entangles souls in flesh) and “correcting” it with his stoic belief that procreation is only “excuse” for intercourse.
Gnostics didn’t use NFP in order to satisfy lust. They used it because of the belief that begetting is evil act, and used contraception to stop it. Augustine claims they got it all wrong. Begetting is the sole reason of sexual activity, and each act done without wish for begetting is “talking a wife for sexual gratification.”
This can be traced in other work of this saint. For example, his idea that genital erection is not under control of ration because of Original Sin.