My husband does the same thing.
It got to the point it was everynight, he was lieing about it, and not having sex with me for weeks at a time.
I was fed up over crying, wishing he'd come to bed, for him to only come to bed around 3am..
After I caught him masterbating to it..
I put a password on the computer, and when my dear ol' husband tried to fake me and lie.. he realized there was a password and he was screwed.
Sounds immature for me to do it - but it worked. I was tired of him looking at that crap, and expecially when I saw what kinda of crap it was (since he's stupid with the computer, he doesn't know to clear history.)
Now he know's I don't trust him at all to get online. And I told him.. "Look here! Password is on, this computer is for me, I bought it with my money, you have your xbox 360.. stay off mine, and i'll stay off yours!"
Needless to say he hasn't been online in almost 2 months. If he don't like it.. Well then he can just leave. LOL (might be my pregnacy hormones saying he could leave, I wouldn't want him too.)
Well somedays I do. LOL