As a man with questionable morals in my fantasies, I can tell you that MOST men won't act on those fantasies in real life. How do I know? Because most men have fucked-up fantasies of some sort, whether they will admit it or not (to others OR themselves)...
You are in a unique situation now. You know about his perversions, and have information on him that you can choose to do with how you please.
1) You can report him to his family, friends, employer, police... This is an extreme case. If he were actually hurting someone with his perversions, or showed a tendency toward it, then this would be an option. As it is, he seems like he's just a voyeur, not hurting anybody. In this case, it would be a bad move.
2) You can use this knowledge as leverage in the relationship. At any rate, you need to let him know that YOU know, but this could be beneficial to you. You can get extra stuff from him, have him do extra chores, whatever. Only do this if you don't mind him eventually getting fed up and leaving after a year or so.
3) You can look at this for what it is, a deep dark fantasy. You can approach him knowing you can share your deepest most embarrassing fantasies as well, and he probably won't judge you. In fact, he will probably be really receptive, and play along with you. This could actually strengthen your relationship and your sex life. It could create a bond that neither of you would likely have with any other person.
Unless, of course, you don't HAVE any deep dark fantasies, in which case, just leave him. Maybe you can find a man that can keep his fantasies hidden better, or at least one that can erase the fucking browser history.
