He has a girlfriend but flirts with me?

[Ella ♥]

We're in college. His girlfriend is hours away in another college. They've been together for about 3 years. In class he's always sitting really close under me, flirting, getting all in my face, and talking "personal". I don't like him at all but I think he's attractive. I know I'm attractive too (so I've been told alot). But why does he keep flirting with me when he has a girlfriend and he's always on twitter saying how much he loves her. I definitely keep away from guys with girlfriends because I'm not like that. He's sweet & funny but when I found out he had a girlfriend he was off limits. What do I do?