Nor does it matter if a man having knowledge of a woman by fornication, make sufficient provision for the upbringing of the child: because a matter that comes under the determination of the law is judged according to what happens in general, and not according to what may happen in a particular case.
Reply Obj. 1: Fornication is reckoned in conjunction with these things, not as being on a par with them in sinfulness, but because the matters mentioned there were equally liable to cause dispute between Jews and Gentiles, and thus prevent them from agreeing unanimously. For among the Gentiles, fornication was not deemed unlawful, on account of the corruption of natural reason: whereas the Jews, taught by the Divine law, considered it to be unlawful. The other things mentioned were loathsome to the Jews through custom introduced by the law into their daily life. Hence the Apostles forbade these things to the Gentiles, not as though they were unlawful in themselves, but because they were loathsome to the Jews, as stated above (I-II, Q. 103, A. 4, ad 3).
Reply Obj. 2: Fornication is said to be a sin, because it is contrary to right reason. Now man's reason is right, in so far as it is ruled by the Divine Will, the first and supreme rule. Wherefore that which a man does by God's will and in obedience to His command, is not contrary to right reason, though it may seem contrary to the general order of reason: even so, that which is done miraculously by the Divine power is not contrary to nature, though it be contrary to the usual course of nature. Therefore just as Abraham did not sin in being willing to slay his innocent son, because he obeyed God, although considered in itself it was contrary to right human reason in general, so, too, Osee sinned not in committing fornication by God's command. Nor should such a copulation be strictly called fornication, though it be so called in reference to the general course of things. Hence Augustine says (Confess. iii, 8): "When God commands a thing to be done against the customs or agreement of any people, though it were never done by them heretofore, it is to be done"; and afterwards he adds: "For as among the powers of human society, the greater authority is obeyed in preference to the lesser, so must God in preference to all."
Reply Obj. 3: Abraham and Jacob went in to their handmaidens with no purpose of fornication, as we shall show further on when we treat of matrimony (Suppl., Q. 65, A. 5, ad 2). As to Juda there is no need to excuse him, for he also caused Joseph to be sold.
Reply Obj. 4: Simple fornication is contrary to the love of our neighbor, because it is opposed to the good of the child to be born, as we have shown, since it is an act of generation accomplished in a manner disadvantageous to the future child.
Reply Obj. 5: A person, who, while given to works of piety, yields to the inconstancy of the flesh, is freed from eternal loss, in so far as these works dispose him to receive the grace to repent, and because by such works he makes satisfaction for his past inconstancy; but not so as to be freed by pious works, if he persist in carnal inconstancy impenitent until death.
Reply Obj. 6: One copulation may result in the begetting of a man, wherefore inordinate copulation, which hinders the good of the future child, is a mortal sin as to the very genus of the act, and not only as to the inordinateness of concupiscence. On the other hand, one meal does not hinder the good of a man's whole life, wherefore the act of gluttony is not a mortal sin by reason of its genus. It would, however, be a mortal sin, if a man were knowingly to partake of a food which would alter the whole condition of his life, as was the case with Adam.
Nor is it true that fornication is the least of the sins comprised under lust, for the marriage act that is done out of sensuous pleasure is a lesser sin. _______________________
THIRD ARTICLE [II-II, Q. 154, Art. 3]
Whether Fornication Is the Most Grievous of Sins?