Not sure .... My therory is they are collecting all the data on what we view and they're gonna black mail us on June 17th of this year. I'm usually correct about everything ..except when I'm not .. So get ready to take out a second morgage you sick faaks :)
Porn sites have some the highest rates of paid user subscriptions. A lot of the make their money too by hosting preview videos for content of pay only sites, when someone redirects from that they get a little money and then a little more if someone signs up from the redirect. Some charge for downloads. There's a lot of horny ass lonely perverts jerking off and and rubbing beav out there.
Then the sketchier small ones make money by hosting malicious code knowing that porn site visitors have an unusually high amount of out of date flash and other exploits.