In a serious relationship but interested in someone else?

I've been with my boyfriend for 6 months, which isn't very long at all, but we're fairly serious. I've considered myself a biual for quite a while, but I've never really liked a girl until very recently. I feel bad because I love my boyfriend but I'm infatuated with this girl I go to school with. I'm pretty sure she's straight though, and I'm far too shy to even talk to her. So yeah I can't even remember what I wanted to ask, but anyways, is it a bad thing to be really into someone else while you're in a relationship? I would probably act on it if given a chance. It would be cheating, but my boyfriend doesn't think of girl on girl stuff as cheating so I guess that makes me feel better. Anyone who's been in a situation like this PLEASE HELP ME!!