Tumblr does not display ads on pornographic content. It is not something that Yahoo is willing or able to monetize, and yet it represents a substantial amount (perhaps the majority) of the cost of operating Tumblr.
Maybe a future operator of Tumblr would feel differently -- but I think it would be difficult to monetize both sets of content simultaneously.
> I think it would be difficult to monetize both sets of content simultaneously.
I'm not really certain why you'd feel that way. Most ad agencies simultaneously have an adult wing and a mainstream wing because the truth is... most people in the world are adults, and sexuality is a large part of what we consider the human condition.
It's only natural that you'd pay for services and products to aid in that.
What's not done is mixing the content verticals. You have pornographic content, then you advertise Adam & Eve products, paysites, or one of the many clothing distributors that will never be seen in a local mall.
Ironically, since tumblr lets' you use your own templates---some people are successfully doing upsells via Tumblr using "in house" advertisements for companies.
Maybe a future operator of Tumblr would feel differently -- but I think it would be difficult to monetize both sets of content simultaneously.