Sorry, but that's absolutely incorrect. No such content is sold in any legitimate form in Japan, and you'd be hard pressed to find any example anywhere. And I'm commenting specifically on your "It's not just fiction" bit, meaning actual humans being taken advantage of in those ways.
Hentai is fiction, and yes there are several types of hentai (always fiction by definition) that contain sexualized depictions of all kinds of things deemed taboo in other societies (from incest to aliens, tentacles and young teens, etc).
I won't derail the thread anymore, but yeah, those materials 100% have the right to exist and it's a good thing that they do. I rather someone with "bad" proclivities to find release in fiction than go outside and molest real people, be it sexually or by committing extreme violence. It's all part of human nature and there's no point in hiding it.