If you have told him you don't like him going on these sites (and honestly, if you are married you shouldn't have to even do that - he should KNOW - and some part of him obviously dooes if he is hiding it from you) and he persists on doing it: Then I am sorry, but in my opinion he is already cheating on you. He may or may not have actually done anything already - but he seems to display a very clear intention in what he is doing.

Step back from a minute and ask yourself: If a friend came to you and told you that HER relationship was like this and HER spouse was acting in such a manner - what would you think and say to her?

Unfortunately I don't see anyway to resolve this if he isn't even willing to admit that he is doing anything wrong.