For some people sex = "cheating" cheating and the like. Honestly, if my GF were to have sex with a dude, I'm going to simply leave her. If I take her back after that is up to her, but it'd be very unlikely.
If she had sex with a girl though, I don't even know how I'd react. But, that's probably because she doesn't even like her own, so it's really hard to imagine.