Double standard: Gay best friends okay, but lesbian or bi friends a no?

Okay, so at my school, there are these girls who hang out with a gay friend. I try to hang out with them but they're always so apprehensive and cautious around me. I'm bi, although I haven't came out to them. The girls suspect that I'm a lesbian though(That gay friend of mine secretly told me that the girls said that, which isn't totally wrong, ha.) I don't get why they're so comfortable around my gay friend like they're best friends with him and they maintain their distance around me. This double standard really stinks.