Girls why is it so bad for us guys to have female friends?

My girlfriends doesn't want me around any girls that I consider a friend the only girls I'm able to hang out with is family. I love this girl to death and we've been going out for a while. I always buy her things on special occasions, I'm there when she needs me or when I know something is wrong and I'm always thinking of ways to demonstrate my love to her. Not only is she my girlfriend she's my best friend and the other girls I hang out with are JUST FRIENDS. Why doesn't she trust me if she knows how much I love her and never will cheat on her?