The thing about a woman and her friends - they are the most important people in her life, way more important than you are. You're never going to compete with them, so don't put yourself in that position. Even if you win, she'll just end up hating you.
The OP's girl is probably just feeling him out to see if he hates gay people. I think the right answer is to say, "Cool. We should all hang out sometime." And that is the best way to make your own judgment of the guy, meeting him in person. As alluded to earlier, sexuality is a spectrum. There are a lot of bi guys. But there are also gay guys who are never going to be attracted to a woman.
My ex wife had one of the latter, a childhood friend who really did sound like a woman on the phone. I was always nice to him. Years later, she and I were fighting because she had gained a lot of weight. All of her female friends said I was an assh0le. The gay guy was the only one who took my side. "Girl, this is NOT what he signed up for" was what he said to her. She took that advice and went on a diet.