Nah, the length should be fine from the site's point of view. I just posted an 11,000 word story, which is well over 20 pages in Word.
As to what the reader wants... hell, I write stories that don't reach the sex until 5000 words are gone. I am DEFINITELY the wrong person to give advice here.
The rule of thumb is.... write your story. Where the sex begins, doesn't matter. But when writing backstory, be sure your setting the scene, introducing the characters and launching the plot... and nothing more. Too much backstory becomes what is typically called "info-dumping"... which is bad. It bores the reader, drags down the story and is tedious to write to begin with. Keep your backstory quick and to the point. Your characters, setting and plot will be your best guides as too how long is needed.
Less is more. Always.
And with erotica, sex is the main interest of your readers, so backstory needs to be a brief as possible, without cheating your story. But get to the sex quickly. You can keep your readers hooked with well crafted lead up as you present the plot, and it's a good idea if you have a lot of backstory you want to put in, to intermix it with the plot leading up to the sex. Never bore your readers if it can be helped. =o)
It's all about balance. Too much sex and not enough back story makes for an equally boring read. A good back story gets the reader interested in the characters and aids in the "picture" of the sex that is about to take place. That's why I build a lot of backstory first, and try to write the sex without the cliches.
I don't like a backstory at the start of a story, I much prefere smatterings through out the whole story.
To be perfectly honest, I like backstory. A lot. It makes a better story. I like stories that take a while to read, and being a speed-reader, that means I want a HUGE story.
Backstory is a must for me but it has to be eroticly charged if that makes sense. For example let's say the setting is a classroom and the players are a teacher and student. I DO want to read about how the lust builds to the point where they are both willing to risk it all for mind blowing pleasure, I do NOT want to read all about the history of Abraham Lincoln lecture the teacher was giving during class. If the backstory is filled with sexual tension I will read for pages and pages. If not I skip down until I see the words, lick suck or fuck.