There are options to specifically tag an avatar as having NSFW content, so my guess is that you can’t be banned for HAVING an NSFW avatar, unless having that tagging option for creators is just a trap - but that’s highly unlikely.
Now as far as a NSFW avatar in a private world, my guess would be ‘it depends’.
If you’re in the private world where there is a reasonable expectation of encountering such things, then probably not. But if such content is being used in a space that is inappropriate for it, then absolutely! I think the your guiding star should be these words:
‘Is it private and are all parties consenting adults?’