Maybe your vision of tumblr's userbase is biased by the kind of people you follow. Personally, I'm on the "computer science/glitch art/study/science" part of tumblr and I don't see much NSFW content (Apart from some bots who follow me which I block automatically).
The areas are kept quite separate, but that doesn't mean that there isn't still a huge porn userbase. Many people on tumblr have multiple blogs on different subjects, and deliberately keep their porn blog(s) separate from the rest.
