You have a medium of anime and manga and even live action where it's a common enough way to depict something sexually without challenging censorship or demographic controls. I mean, it's not like porn where you have to theoretically be an adult to get a hold of it--pantyshots are in WSJ and all. Put on top of it a culture in which men are often somewhat meek, don't have opportunities to be social with women, and so probably still have to greater or lesser degree that little boy's mentality of naughtyness...you get artists who can thrive on risque-yet-not-entirely-taboo depictions.