What to do if you suspect your boyfriend of being on a single's website?

I've been with my live in boyfriend for almost 8 years. I am suspecting that he is always on singles websites looking. I caught him many years ago on one, and he had even placed an ad looking for people in the city we lived in. He claims it was for business purposes, but it was listed under "men seeking women" section. Every time I am asleep he is on the computer, and I do not know what he is doing. He shuts the computer screen off quickly when I come into the room. Also, I noticed that he is "friends" with more women than men on Facebook. Like some random woman will "friend" request him, sometimes even by accident, and he always accepts their request. I always catch him looking at photos of women on Facebook, and I ask him why he is doing that and he gets furious with me, and closes the website. One more thing, he refuses to put on his Facebook profile that he is in a relationship. He just leaves that part blank. It hurts me that he is always on the internet. I suspect him of wrong doing, but I am afraid to look at the history, because I am afraid of what I will find. I don't think I could handle being hurt any more than I already am. I think it would be devastating for me if I were to really look. On the other hand I feel like I am just burying my head in the sand, and wearing blinders to what he might be doing. I don't trust him 100%, but I love him very, very deeply. When he was sick in the hospital with a staph infection a while back, I nursed him back to health. He relies on me for everything, and I think he is used to me being a comfort to him, but I know for sure that I do not turn him on anymore. Those days of good sex are gone. He never makes love to me, and when he does it is very, very lame & quick. He never "finishes" either.