I would break up with her immediately, not because she slept with a girl, but because she cheated. I won't do anything like ask her to invite the other girl over for a threesome, no, that's just encouraging the inevitable because she's going to have to pick one and since she stepped out then it means she wasn't satisfied with me.
Cheating is cheating irrespective of who you cheated on me with. Relationships are like mini-marriages and although you don't take any vows, you're still accountable to your partner. A wise man once said that if you love two people then pick the second person because if you truly loved the first then you wouldn't have fallen for the second person. So if she cheated then that means that either she never really loved me, or she's confused about her sexuality, and believe me when I say you don't want to be with a confused woman.
Provided I'm doing my part in the relationship and satisfying every demand then she has no right to cheat so if she does then she can go off and enjoy her new love life.