Summa contra Gentiles3.126 Chapter CXXVI. That not all carnal intercourse is sinful

Just as it is contrary to reason to indulge in carnal intercourse so as to frustrate the begetting and rearing of children, so is it in keeping with reason to make use of it in a manner consistent with procreation and upbringing. Now, the divine law forbids only those things that are contrary to reason, as we have shown above. Therefore it is unreasonable to say that all carnal intercourse is sinful.

Again. Since the members of the body are instruments of the soul, the end of each member, as of any other instrument, is its use. Now, the use of certain members of the body is carnal intercourse. Therefore carnal intercourse is the end of certain members of the body. But that which is the end of any natural thing, cannot be evil in itself: since that which is according to nature, is directed to an end by divine providence, as stated above. Therefore carnal intercourse cannot possibly be evil in itself.

Further. Natural inclinations are implanted in things by God, who moves all things. Therefore the natural inclination of a species cannot be to that which is evil in itself. Now, in all perfect animals there is a natural inclination to carnal intercourse. Therefore carnal intercourse cannot be evil in itself.

Moreover. That which is a necessary condition for something good and very good, is not evil in itself. But the preservation of the animal species cannot be enduring except by means of generation by carnal intercourse. Therefore carnal intercourse cannot be evil in itself.

Hence it is said (1 Cor. vii. 28): A woman sins not if she marries.

Hereby we refute the error of those who say that all carnal intercourse is unlawful: wherefore they utterly condemn matrimony and nuptials. Some of them are led to this assertion because they believe that bodies were made not by a good but by an evil principle.