there was a korean sex mmorpg but it failed.
there are mods for NW2 for some sort of semi-adult mmorpg feeling, with focus in RP.
also tehres this tool called free adult chat or wahtever its called, features 3d models. you look for partner online and then smecks them. similiar to 3dsexvilla . you need to pay for teh good clothes and stuff for. girls get free points once they paid, at least they pamned that.
also thres redlight district.
they all fail
in short, a sex mmorpg would be a waste of time, as its just sex based. would get boring quickly and there are just not enough ppl who would pay for it to warrant the expenses an mmorpg would bring with it.