Earlier quoted context omitted.
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…
Ah, that makes sense. I was seeing it from the point of view that reblogs are seen as just that. A method of sharing, not a method of laying claim. Its a way of saying "Look at this thing, and heres the source" thats pretty simple to do. To me, "Content" implies stuff you uploaded directly, being the source yourself. I can see the confusion now.
You'll sometimes see someone tag a picture post, if they tag it at all, if it's something the blogger actually created themselves with #oc for "original content" or they'll just tell you as much in the caption. Not always, but sometimes.
I'm not on tumblr for the porn (I read it for the articles, I swear), but a lot of the mechanisms and meta-memes of how people use the platform are pretty consistent across subgroups.