To spread backdoor trojans and other unwanted scamware to people's computers. That said you get stuff like that from google's searches not even related to porn too, I did stumble onto a previously not black listed link trying to do my day job not long ago. Unfortunately I did get infected badly too, only then did I learn the Java zero day exploit that got so much media hype not long ago won't actually get addressed until an update expected to come out February 19th 2013.

....Which I expect is news to most here as the they and the media said everything was fine from Java 7 update 6 - but even the rumor on the nerd hotline that an actual fix wouldn't be in-place until update 9 turned out wrong - so a source for that is required:
[link to blogs.computerworld.com]
«The flaw that puts a billion users at risk won't be patched until February 19, 2013.»

So yes, even if you have update to Java 7 Update 10 you are still exposed if you have java installed, so do visit free porn sites with your shields up as well as take care in your google image searches in particular. I recommend installing WOT (Web of trust) into your browsing habits.
[link to www.mywot.com]

Not all porn site are bad. One reason there is many sites is porn generates many clicks by nature and many draw advertisement income and run a straight operation, apart from perhaps stealing content and cheating on the clicks with an endless list of similar mirror sites tied to the same host which sends you in circles before you get what your eyes desired. But yeah in-between all that, it's like picking a lottery ticket, a loser's ticket that is, on who is out there just to infect you. Fap with backup I say.