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.