I wish the could just tag particular subtumblrs as NSFW or something and filter these by default. Users should be let to choose for themselves. Banning adult content is ridiculous.
There is already forced login for tagged adult subs, as well as a quarantine mode that embed the affected subs to your timeline rather than accessible as its own subdomain. If anything this has already killed much discoverability because search engines are prevented from crawling and their in-house search is useless anyway.
I believe that those systems are currently in place, but either aren't considered to be "enough", or people aren't using them properly.