Chapter 29 — Part III: The Decalogue — The Sixth Commandment
Section 15. Impurity Is A Filthy Sin — The above-mentioned calamity is indeed common to every mortal sin. But what is peculiar to this sin is that fornicators are said to sin against their own bodies, according to the words of the Apostle: Fly fornication. Every-sin that a man doth is without the body; but he that committeth fornication, sinneth against his own body. The reason is that such a one does an injury to his own body violating its sanctity. Hence St. Paul, writing to the Thessalonians, says: This is the will of God, your sanctification; that you should abstain from fornication, that every one of you should know how to possess his vessel in sanctification and honour; not in the passion of lust, like the Gentiles that know not God.