Bi-sexual girlfriend - what do guys think about that?

It's hard to say, because every guy is different.
He might like it, seeing as how you are not likely to cheat with another guy.

Me and my girlfriend have been together a long time, and I've always had my fantasies about her and another girls(threesomes). It took me a while till she told me she is sort of attracted to that. So, we have our fantasies, and phone sex sometimes about that. But to actually do it! It's kind of hard to take it seriously. Yet in school she tells me that she checks out girls, or that she is attracted to them somehow, but not sexually, according to her. The thing is that I feel mad at that and in the same time it turns me on. I know its my fault, she warned me, but it's just fantasies. It's fun thinking and maybe doing too right, but I know ill jeopardize our relationship. Do you think she is turning bi on me?

My ex was bi, it didn't bother me until she started trying to see other people (including women) while we were still together. That ended the relationship pretty quick.

There's not a feeling quite like a girl turning you down because she likes another girl, but if it's a girlfriend, it would be related, but still different. Threatening, maybe, because now he has THE ENTIRE HUMAN POPULATION OF THE WORLD as competition. But I guess threesomes could be a resulting fantasy of that, and it depends on the guy, really.

I'd think "That's why we have so much in common, she likes women too!" I'm not religious and would be cool with what she enjoys. It's weird to me that a guy could think differently, considering guys are so creepily kinky sometimes.

I had a girl cheat on me for several months (I had no clue of course) and the way she did it was pretty immasculating to me. I didn't like that one bit.

But, then again, I'm a Christian who believes marriage can be a great thing and I've been for long term (one on one) relationships so that shapes my view of things. Similar to what Shanekokayne said, it depends on the faith and belief system he has.

It kinda depends on his religion.

He could become insecure, even if it is a turn on. Make it clear that you are committed to him, despite your sexually ambivalent feelings.

I would be really turned on knowing that she likes women and would hope for the threesome.

He'd either be turned on and hope for a threesome with another girl or get really worried that you'd leave him for a another girl. I had 2 bi girlfriends and one left me for a girl and that hurt, the other got me the threesome and it was awesome. Just let him know and if he leaves you just find someone better.

Okay so I am bi so I can straight out say that some guys are cool with it and others aren't. Some can't stand it and wanna fight and argue because you like girls. And there are guys that think it means they can mess around with girls because you are. And then there are guys who just couldn't care less if you do or not.