Where online can I submit fictional sex stories?

There are many:

As has been said ASSTR at http://www.asstr.org/ is one. ASSTR gives you your own individual Web space to post anything you want to that becomes part of the overall ASSTR site and that that the public can view for free, but you have to code your own HTML to link to your stories. ASSTR is run by volunteers who care about making a home for amateur stories. They're good guys (and girls)!

Then there is StoriesOnline at http://storiesonline.net. All you need to do is sign up for an author account (free) and submit your stories. You can join for free as a reader, but some parts of the site are closed to free members. After you submit so many stories, you get upgraded automatically to the level of a paid account for free--permanently. SOL has an elaborate search mechanism that lets you zero in on the stories depicting the particular activities and kinks that turn you on, such as straight, lez, gay, bdsm, rape, incest, pedo, animal, etc (--BTW, before that religious fanatic gives me another bad rating for advocating rape or pedo...they're just text stories from someone's imagination!!! No real rape or pedo is happening--). Being hosted in Canada, it isn't subject to US Censorship of certain depicted activities.

Then there is Literotica at http://www.literotica.com. Again, all you need to do is sign up for an author account, which is free, and submit your stories. The editors are a bit selective, however. They have standards and they don't accept just anything you submit. Also, some topics, like pedophilia and snuff, will be rejected.

If you want to get paid for your stories, there is Ruthie's Club at http://www.ruthiesclub.com, but your writing skills need to approach that of a professional writer. In order to read stories, you need to pay a membership fee. As a writer, your membership is free as long as you submit one story at least every six months. The nice thing about Ruthie's Club is that they will illustrate your stories for you when they post them!

There is also Cleansheets at http://www.cleansheets.com that will pay for stories.

On a closing note: As of this posting, to the best of my knowledge, these sites are all safe to visit, i.e. they won't send you to pop-up hell or sneak malicious software onto your machine or turn your inbox into a spam pit.