Albert Barnes Commentary Matthew 5:27-28

"Ye have heard that it was said, Thou shalt not commit adultery: but I say unto you, that every one that looketh on a woman to lust after her hath committed adultery with her already in his heart." — Matthew 5:27-28 (ASV)

Ye have heard—Thou shalt not commit adultery. Our Savior in these verses explains the seventh commandment. It is probable that the Pharisees had explained this commandment as they had the sixth, as extending only to the external act; and that they regarded evil thoughts and a wanton imagination as of little consequence, or as not forbidden by the law.

Our Savior assures them that the commandment did not regard the external act merely, but the secrets of the heart, and the movements of the eye. That they who indulged a wanton desire; that they who looked on a woman to increase their lust, have already, in the sight of God, violated the commandment, and committed adultery in the heart.

Such was the guilt of David, whose deep and awful crime fully shows the danger of indulging in evil desires, and in the rovings of a wanton eye. See 2 Samuel 11:1–27; Psalms 51:1–19. See also 2 Peter 2:14. So exceedingly strict and broad is the law of God! And so heinous in his sight are thoughts and feelings, which may be forever concealed from the world!