Is porn a relationship killer?

I have noticed that my hubby has been frequenting various porn sites when (and only when) I am not home. He lies about it until I show him evidence of my knowledge, which is so degrading for him, I'm sure. He says he feels disgusting, and that it's a compulsion which he can't control. I am having trouble trusting him when I leave the house, and I know that I might be reading too deeply into this. I blocked the sites that he normally goes to, and I found evidence of him searching for them in our search engine. He couldn't find them, of course, but the fact that he actively was seeking them out and was upset when he couldn't find them shows me that this is a little more than a compulsion. I told him I blocked the sites, and he told me that there were ways for him to get around it. I know there's not because I'm awesome, but his nasty attitude about it rubbed me the wrong way. Could this be the beginning of the end for us if I don't look the other way?