As human beings, I don't think we were ever naturally wired to be that monogamous in the first place. You can't really blame the guy for what he did and I bet many guys in the same exact situation would do the same ****ing thing (literally, the thing they're doing is ****ing).
Anyway, she has a right to know. She'll probably dump your ass though lol. But she does have a right to know.
You'll eventually find someone better anyway, and if not, that's your punishment for being a cheater.