Short answer: they had it before you did. That's what happens when you decide you like a symbol that already has widespread recognition as being associated with a particular minority or specific belief/lifestyle. Ask any feminist who "reclaims" words that are traditionally seen as sexist.

Longer not-answer: Slashing girls would make me gay? How exactly does that work? I'm not quite seeing the connection between "Hey, those people have chemistry" and "I want to have sex with members of my own gender"...

Are you assuming that all slashers make up pairings on the basis of "it's hawt", so finding two girls hot makes me gay? (And how does that explain straight men, the main consumers of girlxgirl porn?) Or that slashers are really writing self-insert threesomes? Or that "I like this story" means "I am aroused by this story"?