Why do men watch porn?

Why do men watch pornography?
I can understand if they watch it when they are younger, or single. But what about men in a relationship or married? Even with children?

My boyfriend has an OBSESSIVE amount of pornography. I’ve tried very hard to be understanding of this. I’ve looked up way to cop with it online, but I still can’t get a grasp on it.
My boyfriend and I have sex almost EVERY day of the week, and some days twice. I do this more for him then myself, for I haven’t even had an orgasm. But it makes me happy to make him happy.

I’m not in a horrible relationship, in fact, I’m trying to be understanding to make it better. But some days I just want to tell him to delete the porn, watch it online, and just stick with me. He’s told me he is attracted to me, and that I am the most beautiful girl to him, but why in the world does he have all that porn!?