The law would probably surmise that we are all responsible for our own behavior and conduct in public places.
eg- If you go onto a forum, and make threats at someone, its no the host who is in the wrong, it is the person who makes the threats.
Like-wise, if anything dodgy is going on in a game, it is the perpetrator who is responsible for their own conduct.
The server host, may, be obligated to be lawful, and assist in any criminal investigations that occur from events on the server.
eg - If it turns out some Paedophile was trolling your PW server , looking for twinks/kids to groom, then yes - the lawful thing to do, is to either report it yourself, or assist in the criminal investigation should the police come knocking.
Logs etc.
As far as Server Content goes -
I dont think it should really be restricted below a 14.
Neverwinter nights, and all modules created via the toolset, are considered artistic expression, even if they are wonders of modern scripting technology etc.
When I was in high-school art class, the mature thing to do, is not to shy away from adult themes as long as its in the interest of art.
As such, Nudity in games, as long as tasteful, and for the interest of art or story telling, is alright in my book.
Note - Some exceptions to this rule do apply - See 'The Human Centipede - Part II' for example.
That movie has been banned from the UK, because it lacked any artistic purpose what-so-ever, and was considered to have sadistic, sexual themes, with no artistic reasoning behind them.
Basically, Sadism for the sake of Sadism.
Putting Nudity in a game, for the sake of having Nudity, is pointless, and turns the game, into porn.