Groped by a mo bro

I'm not one for condoning violence, but last night I was dancing in a club and a guy (he happened to be sporting a tashe) grabbed my bum!, my instant reaction was to turn and give him a slap! I slapped him hard right on the shoulder and it stung my hand so I'm guessing it stung him! but my question is....Is there some unwritten rule in a nightclub where it's acceptable to grab peoples asses? Should I be flattered?