William Burkitt on Matthew 5:27

Our Savior next proceeds to explain the seventh commandment, which forbids adultery; by which the Pharisees understood only the gross act of uncleanness, and carnal lying with a woman. But, says our Savior, Whosoever secretly in his heart desires such a thing, and casts his eyes upon a woman in order to such an act, entertaining only a thought of it with pleasure and delight, he is an adulterer in God's account.

Learn, That such is the purity and spirituality of the law of God, that it condemns speculative wantonness, no less than practical uncleanness; and forbids not only the outward action, but the secret purpose and intention, and first out-goings of the soul after unlawful objects.