Why is he so private?

I'm 30 years old with a 13 year old daughter. My boyfriend and I have been together for 5 years. I have always had a problem with trust due to prior relationships. I always questioned him and went through his e-mails and phone. I never found anything until about three years into the relationship. He started talking to a co-worker for awhile and then finally one night he went out with her and came home with a hicky on his neck. He apolized and swore up and down it was the biggest mistake of his life and it would never happen again. Now though he has changed his pwords to everything and tells me he will leave me if I continue to be nosey. He just recently got a facebook and like five ex-girlfriends have contacted him. I told him it makes me feel un confortable to have people on there that he had past relationships with and he just tells me I can't control his personal space and I should just trust him. I have seen that a couple of them are sending private messages to him. He reads them when I am not around. He is also sending dirty talk e-mails to a girl he used to talk to. She doesn't live in the same state but, is this ok? He says he is not a private person I just have to respect his privacy. What does everyone think about this. Did I cause this by being so Nosey? I would like some mens advice too please.