Definetly not OK on his part! Even if you were dancing 'provocatively' (which, by the way, you are completely entitled to do! Dance however you want!) then that guy is still not OK slapping your ass. It's your body and he has no right to touch you like that, especially when you are a stranger to him. If you're boyfriend doesn't see that then it's on him - not you! If because a guy decided that he had the right to inappropriately touch a stranger your bf thinks that that's somehow your fault for being a great dancer and having a fun time then he needs to get off his high horse and return to the 21st century. I hope he realises his mistake and apologises, otherwise I think you are better off without him. Hope everything works itself out!