Serious Girlfriend just told me she is bisexual - I'm confused

My girlfriend just told me that she is a bisexual. I've been away for 6 months, and we've actually been kind of on/off during this time; but I came back and the same sparks started flying. After a few days, she admitted to me that she developed a crush on colleague (female) about a month, and that she now is a bisexual. She is very in touch with herself and seems to be very sure of this. She wants to stay with me, and is willing to be in a monogamous relationship - but I feel that now I'd be holding her back from this new found identity.

She hasn't ever had a sexual experience with a female, and if she was in a serious relationship with me, probably wouldn't. I don't think I could handle her being with someone else (just as she couldn't handle me being with someone else), and we both understand this and have communicated it. I'm just really confused. I've read a lot of other posts on this which has been helpful, but I don't know what to do.

I'm not homophobic in the least, but I must admit that there is a small part of me that is thrown off by her being a bisexual (I know that sounds horrible).

any thoughts at all? Thanks (and please don't say "it'd be awesome for threesomes")