To trust or not to trust my boyfriend?

[Carol]

My boyfriend and I (both 21) have been together for almost a year and we really used to get along.

Well, on to the issue. I knew he had been commenting for a few months on a facebook (female) friend's profile with words such as "very sexy", "so, so cute" and even a "joke" comment in which he says "I want to fap!". The girl is someone who he's never seen personally, but who posts a lot of bikini/sexy pictures to her page. They only have the music tastes in common. Appearently they also changed numbers and phoned each other sometimes and texted often. I discredited the situation for a while, until I could no longer ignore it and decided to address it. I asked him about her and his answer was "she's just a facebook acquaintance who added me out of nowhere". Not happy with the response (I have a sixth sense), I decided to add the girl and ask her upfront. I started a trivial conversation and then asked her about him and revealed that I was his girlfriend.

She reacted immediately and revealed that my boyfriend had been after her, that he'd told her he liked her and telling her that what he had with me "just a minor thing" and that I woulnd't mind him being with other women and that he was willing to cheat since he believed he could like two women at the same time. She also revealed that she knew some awkwardly private details about myself - things my boyfriend told her.

I confronted him and he denied everything. Admits to having had long conversations with her but denies all of this and says that it must have been her misunderstanding his words or saying this out of revenge, since she liked him and found out he was comitted. Admittedly she acted sexually towards him, telling him she was naked, asking him out (which he always declined). We had an argument (our first) and he ended up crying, asking me to stay, repeating the same story over and over: it was a misunderstanding, I never told her that, the other girl is a crazy bitch...

Days later, I found out that they had never indeed seen each other personally since she passed us by in college and looked at us without a word. Later she texted my boyfriend wishing him "the best" in a rather harsh tone and minutes later texted him insulting his physical appearence and mine.

Note: The girl is very hot and seemingly eager to expose her sexuality/body online. I'm not uglier but I'm physically and intellctually very different. People from outside our relationship often describe me as "the perfect girlfriend". They often tell me people like me are "very hard to find". I do lots of things for him, I'm very sweet. I am more intelligent and cult than this girl and I have a lot more common interests with him. I can't see a particular lack in our relationship that would make him seek other women (unless her physical beauty).