Tumblr as a platform exists on shared content, and actively measures, and provides to the user-measurements of "reblogs". In the context of this discussion, so many user account blogs that could be tagged as 'adult content' are full of reblogged sexual images that the account owner identifies with or wants to advertise as the type of sexual activity they are interested in.

Many comments here are pointing out Fetlife as a refuge for some tumblr ex-pats. Speaking as a Fetlife user, it's not hard to see why the 'their' is in quotes. When you see a tumblr profile on someone's fetlife profile it's often described by the profile owner as "you can see the kind of stuff I'm into on my tumblr" with a small ratio of original content to reshared content. Either images from a movie, online porn, or just "reblog" of another tumblr user's content when not the blogger's own images that are being posted.

This isn't me taking a position on adult content on tumblr, denouncing it or condoning it either way-but I think that bit of context might help answer your question.

It doesn't define the entire experience for the entirety of the porographically inclined userbase, but it's relevant.