I'm not 100% sure.
But Tumblr allows you to post mature photos that you yourself created or if of yourself and it's done for "artistic" purposes. Like nude modeling or something etc.
Copy and pasting dick picks that have been shared around the net a million times, and just adding gay porn gifs are not tolerated.
How guys are getting away with it is that if they say all the stuff on their blog is their own material, and it's done for non sexual purposes as like I said for art or for naturism etc., it won't get taken down.
Or there are so many tumblr blogs and they just haven't gotten caught yet.