It wasnt just one time.. http://www.mangago.me/read-manga/hapi_mari/mr/hapi-mari/32/38/
If a woman can slap a man, a man can slap a woman. She was saying some stupid stuff that not only degraded him but also herself. Hence, why he slapped her. There are double standards but maybe try not to be so hell bent on them?
I dropped it here
http://www.mangago.me/read-manga/hapi_mari/mr/hapi-mari/28/31/
I cant, I JUSTT CANT, he is already annoying for me and this... HOW THE HELL TO SLAP A WOMAN???