Set up a porn site that is free. But for every picture you want to see, you have to enter a phrase identifying you as a human.
Have some bots doing stuff on sites that require you to enter such passphrases.
Display to the porn browsers the images that the bots come up with, and then have the bots send back the answers that the humans entered. If the bot gets through the confirmation page, reward the human with porn.
Cheers,
Ben