Why do girls only like girl singers?

I love all kinds of music, my girlfriend however seems to listen to only the mainstream stuff and ONLY girls it feels like. When it's a guy it has to be a hot one she can appreciate with the looks and vocally, on top of that I feel like she'd still support people who are vocally challenged and "hot" but claim themselves to be a singer. I love metal mainly and the heavier the better (also electronic like Deadmau5). I don't care if she ever appreciates mine, but I want her to appreciate something besides Beyonce. She can tell who the trash is and who the truly awful girl singers are (like Kesha I suppose? that's her opinion), but she doesn't give anyone else a shot. I don't expect her to jam to Devil Wears Prada and Lamb of God, but I would like her to take an interest in maybe Tool (Maynards voice is like a fairy having an orgasm) or something like that. I think it's mainly she needs to hear vocals and it's always some epic story about a broken heart..that's what they all love but it's flat out annoying day in and day out. I try to put myself there and be like "Well I suppose metal gives me the same euphoria that you get in a form of emotional connection to this woman.." but the two levels put side by side are a joke. I get such a rush from playing guitar and drums that comparing it to Beyonce it would be like rain forest versus a drop of water. It doesn't mean I can't appreciate, I just don't understand. I don't know, if I can get her to play Call of Duty she may end up with a broader view of music..or maybe an instrument :). Thoughts?