There's a stigma when it comes to writing erotic books making it a whole lot more complicated than other genres. They can often be judged as offensive or degrading even if the erotic element is fairly tame.
There are also authors who see them as a cheap way to make a quick buck. It's no surprise that erotic short stories are popular on the Amazon Kindle and while many are written with passion by an author with talent there are also substandard books that were clearly written for the sake of getting another sale. I don't judge writers for that for in my experience an erotic short story will far outsell a much larger novel, and I wouldn't be surprised if a lot of the authors of erotic books started out writing mainstream stories.
I've read an erotic book that was a trilogy of tales about a woman with a magic futa wand that would temporarily give whoever she used it in an incredibly large penis. This would then lead onto a sex act. The premise of the plot was very simple, and essentially comedy, yet it was clear the author had a passion for her work and didn't skimp on details as to how she felt seeing the giant penis inflate in front of her.
I've read another book about a woman with large lactating breasts who decides to feed her two boyfriends. That is the entire plot in on sentence yet the book was 30 pages long describing even the minutest of details.
Some erotic shorts however tend to be too clinical in details, or incredibly brief, essentially listing a number of sex acts rather than actually describing them. I've also read a detailed story about a woman hiking, meeting men upon her journey, then indulging in casual sex. Strangely this too would mention no more than a paragraph or so about each sexual encounter before the main character would be on her way again.
Personally when it comes to my own writing I've noticed there's a tenancy to feel awkward or embarrassed about writing erotica that doesn't apply to other genres such as horror for instance which may be incredibly violent and disturbing. In some ways writing about a werewolf ripping out someone's guts in a gory frenzy is considered fairly normal whereas a story about two lovers giving each other anal sex could be seen as perverse. Maybe I'm a prude or just easily embarrassed. I suppose it's down to the reader making assumptions about the author's inner thoughts. If someone knew I thought about werewolves killing people they'd know it's just a fantasy whereas if a reader knew of my sexual thought's they'd wonder if I actually partook in such activities.
Most of my stories tend to be set around mythology, warriors and fairies but it's erotica that sells. It's a sad fact but when it comes to publishing ebooks a 10 page erotic short will far outsell a 300 page novel, yet on the other hand it's far less likely to receive a review. My guess is people don't like to admit they read erotic books so avoid reviewing their purchase.
"The difference between pornography and erotica is lighting."
- Gloria Leonard.