Honestly, I'm surprised it took this long. I'm assumed Yahoo would have banned/purged adult content immediately after buying Tumblr.
As many people have pointed out here and elsewhere -the platform is largely used for porn, and once those users are driven out you'll just be left with tumblweeds.
I feel like there's a 50 percent chance that is going to occur to someone up the food chain in the next couple of weeks, and this policy will be quietly walked back.