It’s not that they’re unsalable, it’s that they're unpublishable.
If you're thinking of breaking into erotica, or have already been writing erotica for some time, you may be tempted to up the ante. After all, it can get rather mundane writing the same sex scenes with the same themes over and over again. However, there are a few topics that are so taboo, they’re unpublishable.
If you’re just hellbent on writing the next great father/daughter love, think again. Blood-relation incest is absolutely forbidden. A traditional publisher won’t take it, and you can’t even self-publish it. The closest you can get is pseudo-incest. This is relations between step-siblings and step-parents. AKA: People who are not blood-related.
You’re probably thinking – What!? But what about Lolita? Just steer clear of anything that smells like underage sex. You can probably guess why. The closest you can get is barely-legal or age play. Barely legal involves any character 18 years or older. You’ll never have a problem if your characters are at least 18 years old. Age-play involves an adult acting and/or dressing like a child. However, be aware that age-play is strongly discouraged by most publishers. As of right now, Barnes and Noble, Baker and Taylor Axis 360 and Smashwords accept age-play erotica. Amazon doesn't specify what they do or don’t take. They have a vague statement that amounts to – If you think it’s really bad, we don’t publish it.
Bestiality is human/animal relations. It’s strongly discouraged by most publishers. However, the gray area starts when you start using mythical and imaginary creatures. If you plan to have your cat-people get freaky, you can do it. You can also have intimate scenes that involve dinosaurs, unicorns, dragons and other mythical and imaginary creatures. You just can’t have your human mate any animal that you could go out and buy or find in the real world.
Be careful with dubious consent. This is where one character does not want to do the deed but is partially forced or convinced by another character. It’s best if it’s consensual. Be careful when working with dubious consent. As of right now, Apple books, B&N, Kobo, Baker and Taylor and Smashwords take erotica featuring dubious consent.
If your main character has a cadaver fetish, forget it. Erotica featuring intimate relations with dead things is strictly forbidden. The closest you can get is fantasy, specifically vampires or any creature race that you create that is undead.
When it comes to erotica, your imagination is key to creating the perfect book. Just make sure you don’t cross any lines with your concept or intimate scenes.