It is your choice. Nobody can take it from you. Your girlfriend has very clearly stated that she isn’t comfortable with the state of affairs and that’s what you have to work with. It doesn’t matter if I don’t see a problem and the next commenter believes you have basically cheated. What is and is not appropriate in a relationship is purely down to the people involved - and in this case, the person involved aka your girlfriend has said, she isn’t fine with it. That means either drawing clear boundaries with your female best friend or making it clear to your gf that this is non-negotiable, which might well mean, she might leave. Drawing boundaries with your best friend will also effect that relationship, so again, this is your choice and nobody can make it for you. You need to figure out what you want.