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 common goods depend on the law for their
determination, as stated above (I-II, Q. 90, A. 2), it follows that
this union of man and woman, 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 (Suppl., Q. 50, 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 good of 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.

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 th