if your girlfriend has a girlfriend, then she might not fully understand the concept of a faithful relationship, and is clearly Bi-Sexual. just let her know easily how you feel about that, and ask...
A simple answer .. When the time is right. Do not rush into a relationship because you want to have a girlfriend. You will find her when you are supposed to. "What's meant to be will find it's way."...
Just enjoy her company, be friendly and faithful. Treat her with respect and as a partner in the relationship. Words won't do it.
Don't just charm her and make promises you aren't willing to keep....