St. John Chrysostom, on Jesus’ condemnation of lustful looks in Matt 5.28:
“For this cause the blessed Job [Job xxxi. 1] also laid down this law from the beginning, blocking out from himself on all sides this kind of gazing.
For in truth greater is the struggle on beholding, and not possessing the object of fondness: nor is the pleasure so great which we reap from the sight, as the mischief we undergo from increasing this desire; thus making our opponent strong, and giving more scope to the devil, and no longer able to repulse him, now that we have brought him into our inmost parts, and have thrown our mind open unto him. Therefore He saith, “commit no adultery with thine eyes, and thou wilt commit none with thy mind.”
For one may indeed behold in another way, such as are the looks of the chaste; wherefore he did not altogether prohibit our seeing, but that seeing which is accompanied with desire. And if He had not meant this, He would have said simply, “He who looketh on a woman.” But now He said not thus, but, “He who looketh to lust,” “he who looketh to please his sight.”
For not at all to this end did God make thee eyes, that thou shouldest thereby introduce adultery, but that, beholding His creatures, thou shouldest admire the Artificer.” (Homilies on the Gospel of St. Matthew, Homily XVII)