What does Matthew 5:28 mean?

Matthew 5:28 KJV
But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.

Matthew 5:28 NKJV
“But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart.

Matthew 5:28 MKJV
But I say to you that whoever looks on a woman to lust after her has already committed adultery with her in his heart.

Matthew 5:28 KJV 2000
But I say unto you, That whosoever looks on a woman to lust after her has committed adultery with her already in his heart.

Interlinear KJV

But /de/ I /ego/ say /lego/ unto you, /humin/ That /hoti/ whosoever /pas/ looketh /blepo/ on a woman /gune/ to /pros/ lust after /epithumeo/ her /autos/ hath committed adultery /moicheuo/ with her /autos/ already /ede/ in /en/ his /autos/ heart. /kardia/

British Family Bible

whosoever looketh on a woman &c. But I say, Let not thine heart or thine eye commit adultery; for here it is that sin begins, and here it must be crushed. This is wisdom, this is morality, in its most perfect form, in its essence, and in its first principles: bad thoughts quickly ripen into bad actions; and if the latter only are forbidden while the former are left free, all morality will soon be at an end. Our Lord therefore, like a wise physician, goes at once to the bottom of the evil; He extirpates the first germ and root of the disease, and leaves not a single fiber of it remaining to shoot up again in the heart. Bp. Porteus. A most striking proof of the superiority of that morality which is taught in the New Testament, is the great stress which is laid by our Saviour on the regulation of the thoughts: see Mt 15:19; 23:25,27. There can be no doubt with any reflecting mind, that, in order to regulate effectually the propensities of our nature, the check is to be laid, not upon the action, but upon the thought. And, as to this matter, the judgment of our Saviour is decisive. He makes the control of the thought essential; with Him, internal purity is every thing. In speaking of this very declaration of our Saviour, “Whosoever looketh on a woman,” &c. Boerhaave was wont to say, “Our Saviour knew mankind far better than Socrates.” Archdeacon Paley.

Jamieson Fausset-Brown

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).