If we are going by ESRB rules then any game asset that is over 18+ years can count. This means people who are shown naked (nothing covering male and female), doing sex (any and all acts), allow to talk about sex in the game, allow near freedom for you to do such things in the game, pay to get sexual "item", etc ( i cant list more or else i get in trouble).
That should be the M rating for adult games.