I imagine enforcement will be pretty impossible. Looking forward to the first test case.
So, man put on sex offenders register and possibly imprisoned for watching say a 19 year old woman and a 40 year old man having sex, because the woman has pigtails or something? What exactly would qualify legally as 'pretending to be under 18'? Surely in court, anything short of the actress saying something along the lines of 'but I'm only 15' would be impossible to prosecute? Does any porn anywhere do that now? I doubt it.