I wouldn't mind at all. In fact, I'd encourage that. I don't get jealous, and real man don't either, especially
if it's another woman. Now, if she's actually thinking about being a lesbian, that's different. If she can show you the same level of respect as before, then I don't see the problem with having another woman in her life as well.
The great thing about all this is it's a win-win for everyone. Your girlfriend gets what she wants, and you get what you want, which is another booty around and your girlfriend's girlfriend will keep your girlfriend occupied when you don't want to be around your girlfriend.