I see no problem with hentai. A friend of mine watches live-action porn and is into "sharking" or something like that, and he proudly admits he is a pervert, slapping girl's behinds when they walk past.
(that gets him beaten up
)
But when it comes to hentai, he'll wretch in disgust, asking "How can anyone watch that? Its cartoon porn!". The only reason society frowns upon pornography of any kind is because of certain religions. And I just can't seem to figure out why hentai is bad.