Catechism of the Catholic Church §2528 — excerpt
"Everyone who looks at a woman lustfully has already committed adultery with her in his heart" (Mt 5:28).
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.