Quote by sprite
No rule of thumb, actually. i have read some amazing stories in which there is a lot of build up, but the actual sex is a small percentage - i've also read some that are pure sex that are brilliant - mostly, it depends on the author. lol - i don't usually share this in public, just with some of the writers that i am close to, but when i write erotica, i usually write the story with little notes in it: Sex scene here - anal - and then, when i have the story done, i wait until i'm in the proper mood and write the sex scene in. to me, the story is the important part - yes, i want hot sex, but i want it woven into a compelling story. mostly, it's personal preference, Saphira.
That is actually an extremely common method of doing it. And one I usually partake in myself.
At the Original Poster
I would say it is totally story dependent. With the SI/Fi Fantasy genre I feel it is worth noting that there is already a great deal of sex in the mainstream novels, and a portion of the readership tends to look for something more along the lines of story and world building and sex being a side point to the main stories.[except off course int he ones that don't.........] Because in the end sex is the same whether it is in a barn or on a space ship. I and my friends often find that the Sex Scenes detract from really well written long from si fi/fantasy fiction on sites such as this. (Came for the sex, stayed for the plot - like sakura fantasy and spirit and well... you know ;-])
in the end - it is purely the story you want to tell.
If you want to tell a quick stroke story, then you need to set it up as such, there is nothing wrong with it, just realize that you do not have the word usage to go into an awesome world building adventure. Focus purely upon the character interaction during sex, leaving the setting as a side note. Unless of course, they are doing it in a public park where the birds are chirping and the little kids are running around playing tag with there doggies. ( as a note I find the forbidden to be the most effective stroke stories to read)
Where as if you are writing an epic romance story, the sex is perhaps the end goal and the actual character interaction is the meat of the story. What I can promise you is that if the story is good enough no one will care if it has sex in it or not. There is nothing more satisfying than reading a good book.
Conclusion tell the story you want to tell, if it is a new sex scene ever 500 words go for it.
I love amazing RPG games. !!