As has been said before, cheating on your boyfriend/girlfriend is, unless the relationship is what you may call "open", a morally despicable act and a severe breach of trust, that could very well end said relationship right then and there.
That said though, I think a lot of guys would be very much inclined to disregard any such act if the cheating was girl/girl.
The same is not true if it was girl/boy or boy/boy.
Lets not kid ourselves, a lot of guys have fantasies about their girlfriends in a Tricycle* with them, or in a girl/girl scenario. Even if we weren't there when the cheat happened, the fact that it did happen leaves the opportunity open for us to observe, or jump on the bike with them, at a later event.
It should be noted, however, that no guy would ever want a Devil's Threeway** with their girlfriends.
Best friend at Vegas, no eye contact... if he paid for / reeled in the girl... but that's as far as it goes, if at all.
If she cheated with a guy, however, it would be a completely different story. "Don't I satisfy her?", "Was he better than me?", so on so forth... Guys just can't have that. Our egos can't handle it.
From the girl's perspective, a boyfriend cheating is never gonna fly. Ever. It's a surefire relationship-stopper.
*Girl/Boy/Girl
**Boy/Girl/Boy