Yet they are mature enough to be constantly influenced by violence and gore of the video games?
Sex is natural and positive part of human beings. You know there's a lot of studies about children's sexuality around too. And kids should be receiving sexual education since late years of elementary school, not to mention books, films and tv-shows are full with sexuality - hell, even merchandising has been very sexist for god knows how long
And nakedness =/= sex
It's not like seeing some bare skin forever corrups you or something. It is absolutely ridiculous showing a nipple can bring game's age rank higher. In Europe (and in many other parts of the world) attitude toward nakedness is a lot more natural ad uderstandable.
As for America, well, I blame puritans