Jamieson, Fausset and Brown on Matthew 5:28

28. But I say unto you, That whosoever looketh on a woman to lust after her--with the intent to do so, as the same expression is used in Mt 6:1; or, with the full consent of his will, to feed thereby his unholy desires.

hath committed adultery with her already in his heart--We are not to suppose, from the word here used--"adultery"--that our Lord means to restrict the breach of this commandment to married persons, or to criminal intercourse with such. The expressions, "whosoever looketh," and "looketh upon a woman," seem clearly to extend the range of this commandment to all forms of impurity, and the counsels which follow--as they most certainly were intended for all, whether married or unmarried--seem to confirm this. As in dealing with the sixth commandment our Lord first expounds it, and then in the four following verses applies His exposition (Mt 5:21-25), so here He first expounds the seventh commandment, and then in the four following verses applies His exposition (Mt 5:28-32).