My sister used to be bi... and she went out with a girl, and she lost a bunch of her friends, my parents didn't approve of it, and everyone started to treat her differently. It got really bad, and she got really upset about it. The girl that she was going out with was only "bi" because she wanted attention. She got in trouble with the principle because she was bi and she was going out with a girl! (she was going to a Catholic school at the time (Not implying that all Catholics are like that))And I, her sister, got confronted about it when my sis and I go to different schools! (people found out because of facebook)
It takes a brave person to "come out" like that.
Anyway... So what if she gets made fun of? And you get made fun of because your friends with her? She's a good friend right? If she's a good friend you shouldn't care what her sexuality is. Just ignore the people who make fun of you.
In my opinion, if you abandon her just because she's lesbian, then thats not very good at all. If she tries to go out with you and you say no, and she keeps bothering you, go ahead and abandon her.
I agree with SK... who cares if she's lesbian?