Why does my boyfriend look at other women? And how do I deal with it?

I've been with my boyfriend for two years, and we get along great. The one problem I have is that he has soooo many female facebook friends. In the past I've caught him having a conversations with a few girls that I thought were flirty and inappropriate. I fessed up to looking at his messages and we worked it out. But recently I saw he made some more new friends and I was curious about who they were. When I asked about it he said people from high school, but I know for a fact that one of them did not go to school with him. He had requested her and I saw from the computer history he was checking out her pics a lot.

I've been hurt in a past relationship from online cheating and I want to trust my man, but at the same time I don't want to be the naive girl who gets cheated on. He is also thinking about moving to the city where he went to high school, and of course that is where a lot of these girls live.

I want to give him the benefit of the doubt and just ignore this whole thing because everyone says that it's a guy thing. But I can truthfully say that I have no interest in checking out other men on facebook.

I want to know how to control my jealousy in these situations. Is it okay to ask who his friends are? I hear about facebook ruining relationships and I don't want it to ruin mine--or risk sabotaging it myself. I just don't think it is fair for him to be looking around when I'm not doing that. I feel good about myself, but I also need that reassurance from him that I'm the only one.