Wow, a whole load of alpha males here. Stupid *****
You wouldn't be able to beat the **** out of the person your girlfriend cheated on because with your state of mind, it'd be nigh impossible to get one in the first place.
The definition is cheating is simple, how far would you let your partner or persay... mother go with another male? If you can stand in a room and watch your mother and girlfriend suck males off and think it's not cheating then go ahead and get yourself sucked off. If you can see them get doggied by the next door neighbour then by all means go rear end the girl next door. If, however you'd feel even the least bit uncomfortable with any of these things... then just simply don't do it.
Otherwise, a few years in the future, you're going to be staring at a divorce paper like the rest of the dumb bastards out there.