My keyboard-jockey response (first thing that popped in my head when I read your question, take it with a grain of salt):
Just tell her that you love her, but that you seriously want an open relationship. Calmly explain that you've always wanted one and that you've made your decision. Tell her that this is really important to you and that you don't really see yourself ever being monogamous for anyone.
She'll dump you or she'll be cool with you sleeping with other girls. Either way, you win.
-Wolf