My bf looks at porn online a lot

I'd love a man's opinion. I have a wonderful bf that is so generous and really my best friend. We've been together over a year. He moved to my town 8 months ago to live with me and my kids. He is a good man and I love him incredibly. I have a problem with him looking at porn a lot. I checked through the history by chance a week ago and was shocked at the amount. I know he does it and I don't make anything about it but I wonder why he resists having sex with me as much as I want but watches so much porn. What's with that? He tells me I'm beautiful and sexy, buys me lots of great stuff, fixes my cars and home repairs, opens my doors and pumps my gas. I have some other worries though. He often doesn't want me to "see" when he is online or yahoo chatting with his woman friends. Again I don't make anything of this. He invites me to sit with him at times and other times he freaks out. I saw he looks up old gfs on facebook lately too. I'm hoping it's just the holidays making him nostalic. Is this just a stupid guy thing or should I worry he is thinking of leaving or even cheating? What should I do? Ignoring it is eating me up inside. I slightly mentioned that I was a little worried one day and he freaked out because he said I accused him of cheating. I didn't at all. I told him I wouldn't blame a woman to be attracted to him and I'd feel better just being able to pop over when he's online sometimes without him saying I spying. He even threatened to leave me that day and told me to never accuse him again. I never try to hide anything from him. I feel I should talk to him about it but I hate for it to blowup in my face again. I would rather know the truth and deal with it. He says he loves me, I overheard him bragging on the phone about how great I am to his aunt, that I haven't met, just the other day and he says he would never cheat on me but I've heard this lie many times before elsewhere. His honesty is one of his best qualities. He's the best man I've ever known but I am bothered finding all this out now. We are looking at rings and everything. Should I get just a clue, do more checking up, push it or let it go? I can't keep a cheat but I'd hate to lose the best thing I've ever had.