My girlfriend has a girlfriend I know about this and I'm willing to take her as she is because she told me she loves me 100%

You are saying but not saying your g/f is a lesbian. It makes no difference really; she can be bisexual and probably means it when she says she loves you. You handle it the same way you would if she wasn't lesbian. It appears she knows what she is doing, so trust it and take one day at a time.

Tick

Your girlfriend has a girlfriend. How would you react if she had another boyfriend?

This isn't about bi-sexuality. Bi-sexuals are perfectly capable of being monogamous. This is about her wanting you to agree to an open relationship.

Open relationships are okay as long as all parties involved are aware and consenting to the arrangement.

I would be concerned that your girlfriend is not being as up front and truthful with both of you as she should be. If she wants to have a relationship with the both of you at the same time, then the three of you need to sit down and discuss what is expected and what the boundaries are. Do not take her word that her girlfriend is okay with the situation, because she may be cheating on her (especially if their relationship pre-dates yours.)

If you can be secure in an open relationship knowing that she is probably telling her girlfriend that she loves her 100% too, then arrange for a meeting and make certain that everyone is consenting to the open relationship. If your girlfriend doesn't like the idea or comes up with all kinds of excuses, then things may not be as you want to believe.

What do you want in a relationship? Do you want a person committed to you or do you want to share your partner with someone else?