It's more of a karma thing for me. I'd like to think of some girl who had a boyfriend wanted to have sex with me, I'd either tell her think twice or to break it off with him first. Give her a chance to do the right thing.
I see what youre saying. Im not saying the other dude HAS too, it just goes to show that this girl is fucked up. BUT ask yourself....if that other dude and you were switched up and he had no idea WHO you were, but knew OF you, I bet the odds are that he'd bang her.