I would not go so far as to say this is cheating. I think a lot of times girls forget that guys are just that.....guys. We have those urges, too and it's silly if you think you don't. Most of these things probably stem from your own insecurity. And most likely he has always looked at it but didn't want to hurt your feelings. He was probably trying to spare you.

I think that, if you don't deal with these feelings of insecurity, you will not ever be happy in a relationship. You need to realize that he is with YOU. If he wanted someone else, he'd go. He wants to be there but he is still human. He chooses to stay at home and masturbate to images rather than actually go out and cheat.

If you love him then this is something you have to work through. If all he is doing is looking at some porn I don't really see the big deal. But if he is looking at things that are terrible (i.e. child porn) then that is a different matter. I think that, instead of turning the blame over to him, I would take a deep look at why you feel so scared and threatened by things that aren't real.