Without any doubt we must hold simple fornication to be a mortal sin, notwithstanding that a gloss [St. Augustine, QQ. in Deut., qu. 37 on Deuteronomy 23:17, says: "This is a prohibition against going with whores, whose vileness is venial." For instead of "venial" it should be "venal," since such is the wanton's trade. On order to make this evident, we must take note that every sin committed directly against human life is a mortal sin. Now simple fornication implies an inordinateness that tends to injure the life of the offspring to be born of this union. For we find in all animals where the upbringing of the offspring needs care of both male and female, that these come together not indeterminately, but the male with a certain female, whether one or several; such is the case with all birds: while, on the other hand, among those animals, where the female alone suffices for the offspring's upbringing, the union is indeterminate, as in the case of dogs and like animals. Now it is evident that the upbringing of a human child requires not only the mother's care for his nourishment, but much more the care of his father as guide and guardian, and under whom he progresses in goods both internal and external. Hence human nature rebels against an indeterminate union of the sexes and demands that a man should be united to a determinate woman and should abide with her a long time or even for a whole lifetime. Hence it is that in the human race the male has a natural solicitude for the certainty of offspring, because on him devolves the upbringing of the child: and this certainly would cease if the union of sexes were indeterminate.
This union with a certain definite woman is called matrimony; which for the above reason is said to belong to the natural law. Since, however, the union of the sexes is directed to the common good of the whole human race, and commongoods depend on the law for their determination, as stated above (I-II, 90, 2), it follows that this union of man andwoman, which is called matrimony, is determined by some law. What this determination is for us will be stated in the Third Part of this work (Supplement,050, seqq.), where we shall treat of the sacrament of matrimony. Wherefore, since fornication is an indeterminate union of the sexes, as something incompatible with matrimony, it is opposed to the goodof the child's upbringing, and consequently it is a mortal sin.
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.