AO = NC-17 M = R T = PG-13 E = G

The problem is people like you equating M with PG-13, M IS R, but due to the assinine double standard that Games have been given by those who beleive that Gaming is JUST for kids, the rating system is assumed to have failed. Even is "hot Sex" was included in the final product, it is no worse(except in quality) than sex scenes in the vast majority of R rated movies.
and with that, I think it should be noted that the Movie Ratings are "Voluntary", as in, the Movie Publisher can rate their movie whatever they want to. The ESRB Rating is forced on any game intended for sale on any medium outside of the Internet, the Developer cannot issue its own rating for the game and it cannot sell the game in 95% of retail outlets without a sanctified rating.