As far as I know, there are many scientific studies that disprove what you've said - and that porn has, in fact, a harmful effect on young minds. Of course there are many other harmful things that we nevertheless accept, like alcohol, cars or cigarettes. So maybe we can, as a society, decide that letting kid watch porn is OK and not worth the alternative (privacy intruding regulations). But arguing that it's not harmful at all is not, I believe, scientifically justified.