You do know the only reason this game is M-rated is because of "sixual Themes" which aren't actually in the game, right?
The game was rated M because of a third party mod you must download, not because of the vanilla content.
"Originally released with a rating of T for Teen, the game has now been rerated M for Mature, due to "more detailed depictions of blood and gore than were considered in the original rating, as well as the presence of a locked-out art file or 'skin' that, if accessed through a third-party modification to the PC version of the game, allows the user to play with topless versions of female characters." -- Gamespot
http://www.gamespot.com/news/6148897.html