You have lost the plot big time....and you ignored my question (though we already know the answer because you have said that it is puritanical to withhold porn from 13-18s).
Pretty radical.
What, you think "you support some fairly commonplace thing so you must support every extreme variety of the same thing in all cases" is a stupid game? Well, I completely agree. But it's the stupid game you came here to play.
I think there's no harm in a seventeen year old looking at an old Playboy centerfold (clearly porn by your definition), so you immediately ask about thirteen year olds and bukkake. And then, you refuse to specify and explain the harm, even though you've chosen an extreme case to make the harm seem more severe and apparent. So tell me, what happens when a thirteen year old looking for something to masturbate to (because they're not going to just stumble on bukkake doing their homework on the Internet) finds some bukkake?