Oh the subject looked epic and the post didn't disappoint, it's always fun to wallow in someone else's misery.
Sorry, had to get that out of the way, as a pretty open minded guy I'm usually down to try anything once but if you weren't comfortable then she should get mad you for it, especially if the other guy wasn't down for it either. I guess I need more details to be able to accurately gauge the situation. It never hurts to cross out of your comfort zone every once in while, but bottom line is if you are that uncomfortable and this was a deal breaker for her then she isn't that committed to you and your feelings anyway. I know it can be hard but you probably need to let her go.