My bf looks for girls online!?

We have known each other for 10 years and been together this past year. We have been living together for a year and we are expecting a child in a few months. He used to meet girls online and hang out with them, mostly though it was just hooking up. I know a lot of the details as at first we were friends. Since we've been together he's done some suspicious stuff like asking ex girlfriends to send him naked pictures, which I found. He has gone out to dinner with another girl he met online and hung out with for only a month (and yes they were 'hooking up') before we became exclusive. He claimed she was a really good friend of his, yet only knew her for a month. So there is the back-story... So yes, I admit, I spy on him. I don't think it's because I'm crazy jealous, I believe I have just cause to worry as I have never done these things in past relationships. I go through his phone, his computer and his laptop. I found the other day, while going through his email that he had posted ads on craigslist looking for girls to 'go out and have fun' It was while we were in Florida visiting family and I came home 2 days before him for work. He claims he was just bored and wasn't planning on cheating, he just wanted to go out with some people. After talking to his family, most of whom I trust I believe that he never actually went out. I flipped out on him though and I can't seem to get over it. It makes me more angry that he maintains his innocence, I think there is nothing innocent about it, even though he never followed up on it.