I've found two possible sites:
http://infinite-story.com/
Doesn't have a search function, so I can't see if they allow erotic or ENF style stories. The banner at the top of the site seems familiar, however. Has somebody linked to stories here in the past? Does anyone have any experience with it before I sign up?
http://www.protagonize.com/
Hmm... I think this is a no. I've typed "ENF" into the search function and came up blank. Tried "Naked" but the stories that come up don't really seem erotic in the least. The nudity is more of an artistic thing.