The vast majority of Tumblr traffic is via logged-in users hitting the dashboard (activity stream of posts from followed users), global tag pages, etc. It's a s

The vast majority of Tumblr traffic is via logged-in users hitting the dashboard (activity stream of posts from followed users), global tag pages, etc. It's a social network, and most pages with ads are displaying content from a bunch of different blogs at once. It's just like the Twitter timeline. Tumblr has already long had a notion of NSFW blogs and content, which are already treated differently than "safe" content. But since the blog network (individual "sites" as you're saying) is a minority of traffic, and not a major source of revenue in the first place, it's not really comparable to isolating mature content on WordPress or Reddit.