Showing them the exit

How about you just point behind them and say, "Hey, is that girl's boob hanging out?", and when they turn to look, RUN!

Or, if the guy was a real dick, you could just say, "You know, I've been thinking that maybe I should try dating girls, and after tonight, I think I'm ready to make that step."

If he asks for your number, you could say, "Really? I thought you were gay..."

Emma