Gregory the Dialogist

If mine heart has been deceived by a woman, or if I have laid wait at my neighbour's door. Though it sometimes happens that the sin of fornication is not at all different from the guilt of adultery, seeing that Truth saith; Whoso looketh on a woman to lust after her, hath already committed adultery with her in his heart. …

Morals on the Book of Job, Book XXI ↗

Thomas Aquinas

In adultery, however, a man plots deceitfully against the wife of his neighbor, and this plot is preceded by a certain deception of heart, as long as reason is darkened by concupiscence, and so he says, "If my heart has been deceived by a woman," referring to the desire of the wife of another. From the fact that the heart is conquered by concupiscence for a woman, one tries to possess the woman desired by any deceitful means whatever, and so he says, "and if I have lain in wait at my neighbor's door," to take advantage of his wife.

Commentary on Job ↗