Some of the most grotesque acts happen because a person lived out their fantasy. I don't know whether the majority of these people wake up one morning and decide to make their fantasy happen or do they have a problem recognizing 'reality'. Each atrocity has it's own dynamics.
It's emotive and it's possibly a dilemma for Amazon. Just wondering what angle others have.
The internet has been cleaned up somewhat over the years, for example domainname.xxx is now available which makes it easy for Nanny-Net for example. Maybe there should be something similar for self published books.
BTW I am not against seriously erotic stories, I've got quite a few of them, including most of the classics - my problem is directing the subject towards children. A comic wouldn't crack a joke about a sexy girl in a gymslip anymore, but years ago they did. (E.g. UK's Benny Hill). Times are achanging.