In my experience, and from what I asked a few friends about: We have never seen an actual "NSFW" avatar in our combined hundreds of hours of playtime, and those that use the avatars that comes close to being NSFW are usually the younger audience. It is possible to filter out avatars from the world space, and only be able to see your friends avatars. Making your own instances (servers), you are able to kick out players that misbehave.

But I would like a NSFW-tag on content.

And as a side-note: The game is free everywhere, Steam, too.