The click through for non-porn ads besides porn is, in my experience, almost non-existant.
Found it: https://blog.eat24.com/how-to-advertise-on-a-porn-website/
But Mattel, Sony, GM, Apple, and American Airlines aren't signing up anytime soon. That's the problem for Tumblr.
I think this is just one of those instances in which a customer just starts costing you too much money. Will banning the low-rent crowd make a difference at this point? Who can say really. But keeping them pretty much ensures you will not get access to the deep pocketed advertisers.