Why does my boyfriend subscribe/view /dating websites that I do not know about?

This has been an ongoing problem. He is very forgetful and sometimes leaves his e-mail and social networking sites signed in when he gets off of my computer; not to mention the recent history being packed with adult websites, however this is not the part that really bothers me. One time, I found that he had set up an account to a website that is for people who want to have affairs - I was obviously upset and confronted him about it and he felt really bad about it. He even logged onto it in front of me to show me that he had not used it, and sure enough, the account was only a few days old with no mail/friends/recent searches, etc. and when we were done he deleted it immediately. After that incident, he has been more careful with covering his tracks (i.e. deleting the history and e-mails), yet he still leaves them open for anyone else who comes along. I have confronted (not in a threatening or crazy way) him in the past and tried to talk to him about it but he just denies that there is any problem with our relationship and that he does not care about that stuff and told me 'dont worry about it.' He also denies the fact that he is constantly searching for and befriending extremely pretty girls online - even though I have already discovered that as well. Im not crazy and I dont want anyone to think that I go looking for problems or things to fight about - I am usually a Iaid back person and I dont want any of this! I know that this seems like a HUGE invasion of privacy however, I think many people in my situation would look too if their significant other just leaves it open/signed in from time to time. The weird part is that he has been signing up for other sites that are less popular, and some unheard of, as compared to myspace and facebook (the ones I am on). Despite all of this, I do love him dearly, and that is why this is making me crazy and perhaps, somewhat blind to what might be going on. Is he trying to tell me something? Is he thinking about cheating? Is he just 'looking' and fantasizing about women that he deems 'unattainable'?