78. That to commit adultery also means to engage in obscene acts, to speak lasciviously, and to entertain filthy thoughts is clear from the Lord’s words in Matthew:

You have heard that it was said to those of old, “You shall not commit adultery.” But I say to you that whoever looks at (another’s) woman so as to lust for her has already committed adultery with her in his heart. (Matthew 5:27-28)