My individual erotica stories rarely top 3000 words, and I've never written one longer than 5000. I mostly write about 2000 words, which for those in the studio audience, is about eight doublespaced pages of Courier font with one-inch margins. Partly, that's laziness: the fewer words I write for a set fee, the more money per word. It might not make any difference in how much time I spend writing, but it's a mechanism for fooling myself into thinking I'm being paid well for my time. And, if I go on too long, the reader will expect more than one sex scene, and those take a lot of mental energy, and why write two when one is all that's required?
Partly, my stories are short because I really don't think they need to be longer. Someone reading erotica doesn't want to see too much worldbuilding, or at least the editor who's paying for the erotica usually doesn't. They want an intense sex scene, so that's what I try to give them.
The trick in being short and intense is, I believe, lies only partly in the strength of the prose style. Characterization is key.
I can hear the questions now from my imaginary audience--how do you establish a character in 2000 words and write a sex scene that is different enough to grab an editor and, later, a reader, oh, and also have plot?
In short erotica, I feel the plot is mostly a given. 1) The characters meet and exchange pleasantries (if they already know each other, we learn why they're together now); 2) the characters fuck, with or without conflict beforehand (that's Plot conflict, not necessarily personal conflict); and 3) the characters Climax. (Yeah, yeah, it's funny to say "climax" with two meanings...I know, I know, get over it....) Optional ending 1) afterglow; 2) potential for future fucking.
So, on to characterization. I think, for this sort of story, characterization reduces to want versus need. If you know what the character truly needs, and what they only think they need, you've got automatic plot, because usually there's conflict between what a person wants and what they're actually going to get. Just knowing that one fact (well, maybe it's two facts) can be most of the plot, especially if you're sticking to a single point of view. And in a story of 2000 words, it's generally wise to stick to a single point of view.
The second character (and third, if you're ambitious) can then be created simply as opposition to the first. His or her dialogue and action are there to thwart the pov character and, since this is erotica, eventually resolve into collaboration/compromise, resulting in Climax.
Simple example: POV character Joe is lonely, but afraid to sleep with Mary. So he needs sex, but doesn't want it. Which leads me to ask why? Mary is really experienced, and Joe isn't. Mary's opposition could be that she both wants and needs sex with Joe. Why? Because she's been in love with him for ten years, but thought he didn't want her, but now can't wait any longer and wants to push him one last time in the hope of achieving a lasting relationship.
I now have a starting point for the story. I only need details that link into and further that conflict. I don't need to say that Mary likes to wear New Balance running shoes; I can use that word count to have her grab Joe in desperation and then tenderly brush his lips with hers, when he was expecting her to stick her tongue down his throat. Am I making sense?
I always think I've really quantified something when I write it down like this, but in reality I'm not so mechanical about it. A lot of this activity takes place in my backbrain, and the development of plot and characterization and the substance of the sex scene are intertwined tightly like, like a ball of twine. Caveat emptor.