"Why would an atheist care if their kid watched porn or not?"
I beg your pardon. What makes you think that atheists don't care about their children? How bizarre.
I would worry more about imparting that "godless" part on them, just in case you are wrong. That influence on the kids from sex would be less trouble than eternal damnation in hell. But hey, I don't really care either way. I just thought it ironic that one could not care about the Creator but worry over Jenna Jameson giving a blowjob.