I remember watching an episode of Jeremy Kyle. A girl on it suspected her partner of cheating coz she found one of those sites in his history. When he came on stage, he was really laid back, laughing, joking, saying how him and his friends were just taking the p**s out of the website. It was fat women looking for sex apparently and they all got a great laugh off it. The problem was he registered on the site and did give his name. He swore he had no bad intentions-just messing around-that he would never cheat on her. To be fair to him he didn't look guilty at all. His story added up and he passed the lie detector test.
Sometimes men do stupid things that really don't mean anything but do look bad. If your bf was only on those sites once, didn't give details about himself, didn't flirt or even talk to anyone and has never been on them since-then I really think you have nothing to worry about. If he was interested in NSA he would have went on those sites everyday-they are addictive to people who are into that type of thing. He wouldnt have looked once and not signed back in again for a month.
"Don't ask a question if you can't handle the answer".