I've been with my girlfriend for two years. I told her on our first date that I was bi. I also told her without telling her that if that was a problem, then we were a problem. I can't say there haven't been some bumps in the road--at first she wanted me to tell her before I met with a buddy (I'm really selective and safe and non-promiscuous with guys) and then, after I told her it felt like I was filing a report or submitting an application, she eased up on that and said she wanted to know, generally, if I had a friend or not. We're long distance, which helps. We're committed, but we've no interest in living together. I think if we lived together it would have been harder, as I might have ended up lying to her and I've never lied to her. She thinks it's hot, my being with a guy, and has asked for videos of me giving head (I complied with her request) which she watches when we're apart and sometimes when we're together. She is the third woman I've been with in a row who thought m2m sex was hot. I was married to a woman who knew I was bi but didn't want me to act on it. That marriage ended years ago and while I understand not wanting to mess up good thing and am not judgmental about people's situations (my "buddy" is married and his wife has no idea) I just can't be in a relationship where I have to hide who I am and what I like.
I've found, quite a few times with quite a few women, that even if they don't want to know about it or don't think it's a turn-on, they accept it. Maybe it's because I like free-thinking liberal women. I don't know. I just know that secrets, while hot, are not that good for me.
Good luck with your girlfriend.