End Goaling in Writing Eroticabysr71plt©
Most all erotic stories are some variation of "he and/or she stuck/enveloped it in/around her and/or him, and she and/or he moaned in pleasure." Whether or not you can write a story worth posting depends on your imagination in finding new "hooks" (the unusual/unique concept that make your story interesting in relationship to what's generally available) and twists (surprises that arrest the reader's attention) and interesting and arousing circumstances and locations. For an erotic story, it also helps if you can write it in a way that gets the reader at least aroused sexually, and ideally very wet—and, best of all, motivated to get up from the computer and go stick/envelop it in/around her and/or him until she and/or he moans in pleasure (and then stick/envelop it in/around him and/or her again).
In experimenting with writing, devise in your mind some sense of plot—where the story came from (without getting bogged down in background) and where it's going and what it's going to do while it's getting there. Also think of the hooks and twists that are going to make it worthwhile writing. Then, put yourself in the place of one of the characters and tap it out on your computer. You can always go back and change and polish after you get something down—you also likely will come up with new, better twists and hooks while you write.
Do you just walk up to a stranger—or have a stranger walk up to you—and say "Let's fuck," and that's how it starts? Never, or hardly ever, of course. And a story starting this way will fall flat into the "B" ranks as well. You need to spend a little time and effort—but not too much—in making the "contract" believable to the reader.
This is basically your fantasy at work. If it's not making you hot and aroused as you write it, it probably won't do that for anyone else and isn't worth pursuing further. And if it does make you hot and aroused, there's 90 percent justification for having written it in the first place. Go clean yourself up and start imagining the next turn-on story.
Read the story aloud. If the flow is awkward or if phrases stick out or words are repeated noticeably, this isn't good storytelling. Go back to Go.
Although there will be legitimate arguments about this, I suggest that you not spend too long getting to one of the good parts (and don't have just one good part). Quick gratification is the most popular in Internet erotica. If most readers wanted to prolong getting to the first climax, they'd buy their erotica in books, not read it on the Internet. You can play with foreplay for a subsequent climax after you've grabbed the reader with some form of early-on gratification.
And speaking of climax, it's generally important (although, if you are a really good writer, you can occasionally get away with a tease story). The reader is generally seeking climax by reading the story, and you should generally offer it to them. The climax of the story is also important, literally speaking. If it's a good story, it finishes on a twist or some form of closure—with the best stories, in literary terms, not relying on an orgasm to get the story stopped.
It will help greatly concerning whether it's worth posting, if you already have a good sense of sentence structure, grammar, spelling, punctuation, and word usage—the mechanics of writing—and are a naturally good storyteller. You'll lose the reader if your storytelling is bad, even if your plot and the images they invoke are good. Don't rely just on spell check (but use it). Use the dictionary for any word even half way questionable (and contemplate whether that's the best word to use in the specific context). One jarring or awkward word can throw a whole mood off.
A mistake made by many is in using scattered point of view—interchanging from paragraph to paragraph the perspective of different characters. A good story will be told from just one perspective (or will keep the perspectives quite separate; or will break all the rules brilliantly). For erotic stories, the first person perspective ("I"—being one of the characters) is much better than third person perspective ("he," "she," "they")—although harder to write well—because it's much more personal (and a reader of erotica is, ipso facto, looking for something very, very personal)—and a good erotic story puts the reader in the action.
Most important is just to sit down at the computer and do it—taking it wherever it turns you on. You can decide what, if anything, to do with it afterward.