Why make it such a complicated system?
I would simply have a "M" games that can only be accessed by checking a box saying "I am 18 years or older and am willing to see content that is rated mature"
Also I don't see why it is needed to exclude "M" players from the rest of the community, I am sure that they can limit themselves when being in public / "E" places, if they have a place were M content can be expressed.
Specifically, I disagree with
3. “M” players will not be able to join “E” games.
as that is in practice seems to be the same as suspending a player from the site. Even players who don't really mind M content will probably create games "E" as that is the largest player base. Instead I would make it possible to block "M" players from joining your game, using the prerequisites system. (Same way you can have only Members join a game, and prevent Members from joining a game.)
Edited 3/12/2015 21:07:54