If someone hangs himself or kills himself with a dagger, he does not sin against his body but against his soul, on which he inflicts violence. But to fornicate is a sin of the body which touches both the body and the soul. Commentary on Paul’s Epistles.
Ad eos autem pudore afficiendos et reprimendos, qui sunt proclives ad secundas nuptias, apte Apostolus alto quodam tono eloquitur; inquit enim: "Ecce, omne peccatum est extra corpus; qui autem fornicatur, in proprium corpus peccat.".
De his dicit propheta: "Peccatis vestris venundati estis. "Et rursus: "Pollutus es in terra aliena: "
Flee fornication. Because, as Anselm, Cassian, and the Fathers generally teach, other vices are conquered by resistance, lust alone by flight, viz, by fleeing from women, from the objects and occasions of lust, by turning aside the eyes and the mind to see and think of other things. For if you oppose a temptation to some lewdness, or fight against some impure thought, you only excite the imagination by thinking of such things, and then inflame still more the innate lust of the flesh, that is naturally disposed to such acts as fornication.
Every sin that a man doeth is without the body. Does not stain or pollute the body.
It may be said that if a man kills or mutilates or castrates himself he sins against his body, and therefore it is not a fact that every sin distinct from fornication is without the body.
I reply that every sin, i.e, every kind of sins which men commonly and ordinarily commit is without the body. For there are seven capital sins, which theologians, following S. Paul...
By necessity, the former by free will: the latter, thinking that it is sufficient for him that he has not sacrificed, has been deceived by an error; the former, a violator of the matrimonial tie of another, or entering a brothel, into the sink and filthy gulf of the common people, has befouled by detestable impurity a sanctified body and God's temple, as says the apostle: "Every sin that a man doeth is without the body, but he that committeth fornication sinneth against his own body.".
Ye are not your own, for ye are bought with a great price. Glorify and bear the Lord in your body."
Flee fornication.
He said not, abstain from fornication, but Flee: that is, with all zeal make to yourselves deliverance from that evil. Every sin that a man does is without the body; but he that commits fornication sins against his own body. This is less than what went before; but since he had to speak of fornicators, he amplifies that guilt by topics drawn from all quarters, from greater things and smaller alike, making the charge heinous. And, in fact, that former topic was addressed to the more religious, but this to the weaker sort. For this also is characteristic of the wisdom of Paul, not only to allege the great things wherewith to abash men, but the lesser also, and the consideration of what is disgraceful and unseemly.
What then, say you, does not the murderer stain his hand? What, of the covetous person and the extortioner? I suppose it is plain to every one. But since it was not possible to mention anything worse than the fornicator, he amplifies the crime in another ...
What Paul says here obviously applies to the murderer, the covetous person and the extortioner equally well. But as it was not possible to mention anything worse than fornication, Paul magnifies the crime by saying that in the fornicator the entire body is defiled. It is a sin against one’s own self in a way that the others are not.