The contribution of the Anime/Manga culture to pornography is well known, and there are fine books out there on this fascinating subject. There’s very little I could add, but I can contribute with this image:
I found it in the newsgroups, but I guess it comes from one of the millions of related websites. But related to what exactly? Fancifully operated transexuals? Shemale fantasy freaks? Manga mythology? Please note that this image complies with the japanese pornography regulations, as the tip of both penises is blurred out.