Heh, the concept of a man hitting a woman being bad comes from women once being viewed as 'weaker than men' and 'needing protecting'. Personally I disagree with this, maybe because all of the females my age I know could beat the living !@#$% out of me without breaking a sweat...I'm the one who needs protecting xD
So yeah, she slapped him, nothing inherently wrong with him slapping her back.