What's the point, then, of a walled garden if it doesn't have a wall around it? cookiecaper says "There's nothing to fear from a .xxx domain" but if that's the case then, de facto, there's nothing to be gained from it either except a bit of marketing edge. If the argument is for "safety", then it has to be compulsory. And even if it doesn't start as compulsory, the "Think of the children!" brigade will soon start lobbying for it to be made so.

Smarter people than I have written about this before: http://tools.ietf.org/html/rfc3675