You have heard that it was said, 'Do not commit adultery.' But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.
Have you ever really thought about what these verses are saying?
Jesus says that if you look at a woman lustfully, you have committed adultery with her in your heart. Think about that. He doesn't say that if you look at a woman lustfully you have sinned, though you have. He says that you have committed the act, in your heart!
They say that a habit can be formed in approximately 2 to 4 weeks. If you do something often enough it will develop into a habit. Something that you don't even have to consciously persuade yourself to do. That is what a habit is really all about. Habitual behavior can be to your good, like reading your Bible everyday, or praying when a trial comes. If you continue to do the same thing, the habit develops.
If a person commits adultery in their heart on a daily basis, they will very easily commit adultery in the real world. It is their habit. They will hardly even have to think about it.
But even if the opportunity never arises, you are still an adulterer, even if it is only in the heart.