I would be interested to know if the reason why (to put it bluntly) porn in Japan is so extreme is due to their cultural repression of sexuality. The reason I ask is that many Asian countries have similar social stigmas around sexuality, and they also have similarly extreme forms of porn.
Not to mention the existence of hentai, yuri, and ecchi. All of which don't really have widespread western equivalents, despite western cartoons being fairly popular.