I think what UKBOB is trying to say is that if you havent been doing alot of illegal things on your computer that would cause you to be suspected of something, then you don't have much to worry about.

When the Feds find these sites, they tend to shut them down and then go after the people that paid for a mebmbership or signed up/registered for something. The mere presence of your IP visiting that site would more then likely not be enough evidence for intent.

Unless your ISP saw that you had stumbled upon those sites at that EXACT moment(something I would consider rare) and reported it to the cops, I still say that you are probably okay. It's not like this accidentally happends to you on a daily basis, right?