Expositor's Bible Commentary Commentary Matthew 5:28

"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 5:28 (ASV)

The OT command not to commit adultery (Exodus 20:14) is often treated in Jewish sources not so much as a function of purity as of theft: it was to steal another’s wife. Jesus insisted that the seventh commandment points in another direction—toward purity that refuses even to lust after any woman. Insofar as a man looks at the woman with a view to enticing her to lust, he is committing adultery with her; i.e., he makes her an adulteress.