Ask why. I like to ask why when I have friends that say things like this. I don't argue, I just try to understand. Often my suspicions are confirmed but sometimes I learn something.
Discussion
They were claiming that the website was used for child porography - adamant about it. I wasn't in a spot where I felt comfortable discussing it further, and everyone else around us were just rolling their eyes at them anyway. I just let them blow of steam and parrot whatever they had read or heard online.
Good call...