I have a dear friend who is in a marriage that is hard, just plain old hard. And rather than face the pain of "the hard" she escapes for hours in the day into a fantasy world that is starring another man in her church, who doesn't know about how she feels about him. So, she feels like it's safe. But it is an amazing amount of time she daydreams and thinks about this person.

Sexual idolatry could take a turn where it's not fantasy, but something like pornography, and it's got such a grip on you that you know full well it is an idol for you. And you don't know how to get it out of your life, and it competes and it obscures who God is.

Listen to what C. S. Lewis wrote on the subject:

You can get a large audience together for a striptease act, that is to watch a girl undress on the stage. Now, suppose you went to a country where you could fill a theater simply by bringing a covered plate onto the stage and then slowly lifting the cover so that everyone could see just before the lights went out that it contained a lamb chop or a bit of bacon. Would you not think that something had gone wrong in that country and their appetite for food?