"Tumblr however, gets freakin’ filthy. I mean, really, really #NSFW. It’s been said that Tumblr’s secret to success is its adult content. Several years ago, that was probably true. Today, Quantcast’s Tumblr subdomains’ stats show that content has diversified quite a bit. No longer are adult sites the majority of the top destinations on the network. But did it scale on top of porn? Of course it did. And those sites are still out there, if you dig through the subdomain rankings."

Source: http://techcrunch.com/2012/05/12/pornterest-vs-pornstagram-vs-tumblrs-nsfw/

Internet axiom #4382: If it's a great way for showing pictures/video and distributing them there will be porn there. That's not insidious infiltration, it's human nature. There's a reason there are nearly 7 billion of us on this planet. :)

Even within porn, there are rules. For example, no children.

Everyone draws the line somewhere. Tumblr is more comfortable with porn than most.

Tumblr talking about Tumblr.