I've never been in a situation like this either. It's an uncommon one for sure.
I would think of this way:
You're a straight guy, she's a gay girl. Have you ever been with a guy yourself? No probably not. She's never been with a guy either until you. As a straight man, how would you feel if you drunkenly hooked up with another guy? You'd probably feel pretty gross about yourself the next day, especially if it was someone you were friends with. How often would you want to see them after that? By seeing them and having contact with them, you just constantly remind yourself of what happened. It's not that you hate the guy, but you associate him with that bad experience and therefore want to avoid remembering it.
This is how I'm guessing your lesbian friend feels. I've never been in this situation myself, but I can only imagine. I know a straight girl who hooked up with a girl once and she felt disgusted with herself. I'm not saying you're disgusting, but this wouldn't be an unreasonable reaction for her to have.
TheEvilJester is right. You have to give her space. She may come around, she may not. I'm thinking she will because it wouldn't be fair for her to do that to you since you have been friends for so long.
Cheers, brother
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