I don't think seeing 'naked avatars', either just standing around or doing stuff, is the issue. Yup, they are just avatars.....
It's more about chat, on voice or in IM's - you know the kind of 'chat' I'm talking about - and the possible encouragement to take it to skype / cam etc....
Kids will be kids, but some adults will take advantage of them, whether knowing they are under 18 or not - THAT'S what they need protecting from.