Thanks guys, I appreciate the open-minded responses. I think that I should break up with her, not necessarily because she wants to date other men, but because she is unwilling to compromise for my relationship needs in the same way that I've done for her. I think it's only fair that, in an open relationship, limits be set up and agreed upon by all members of the relationship, and that's not happening.
I asked the same questions on "other" websites, and the reponses have been less than favorable. They refer to her as being a "slut" and a "whore", which is fucking stupid because I'm the only person she's ever been with. Just because someone's sexually secure and wants to experiment with other people, ESPECIALLY since they're so honest and upfront about it as my girlfriend as been, does NOT make her excessively promiscuous in any sense of the word. I still love her, and we'll still be great friends, and I hope she finds what she's looking for in an open relationship with her new partner =)