Augustine of Hippo
Catena Aurea · Matthäusevangelium 5
Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
(Serm. ix. 3 and 10.) Thou shalt not commit adultery, that is, Thou shalt go no where but to thy lawful wife. For if you exact this of your wife, you ought to do the same, for the husband ought to go before the wife in virtue. It is a shame for the husband to say that this is impossible. Why not the husband as well as the wife? And let not him that is unmarried suppose that he does not break this commandment by fornication; you know the price wherewith you have been bought, you know what you eat and what you drinkg, therefore keep yourself from fornications. Forasmuch as all such acts of lust pollute and destroy God’s image, (which you are,) the Lord who knows what is good for you, gives you this precept that you may not pull down His temple which you have begun to be.