Dear Girl Lover,
Oh, that's easy! Simply figure out what your girlfriend would have to do in order to make YOU become bisexual. Think about what magic words your girlfriend would have to say in order to make you crave some hot man-on-man action. Then simply use the same method on her! Or better yet, you should go out and get banged by some sweaty man-meat, then come home and tell her about how great it is being bisexual. That's sure to work!
Hopefully you've picked up on my sarcasm by now... If not... Well, that doesn't really surprise me.
Listen up, knucklehead! If she's not into girls, that's the end of it. You can't force someone's sexual preferences. You've already done the very most you can do, which is letting her know that the idea of seeing her with another woman is a fantasy of yours. Now that she knows that, she can choose to do what she wants. Who knows?... Maybe she'll think about it, change her mind and surprise you some day... Or maybe she won't.
One thing is for certain... If you try to push, trick, manipulate, cajole, or hornswoggle your girlfriend into doing something that she is clearly uncomfortable with, you will go from being a Girl Lover to a Girl Loser.