I have been reading Erotic novels for 7 years now, including fan fiction. I've read a lot of good ones and a lot of horrible ones. A good erotic novel is believable characters, plot, and detailed sex scenes. With fan fiction, you have to be familiar with the the fandom that you're borrowing characters from or you're going to have a lot of angry readers. Creating your own characters and writing a plot that makes sense is a lot harder.
I understand what it takes to write a great story worth reading. Also, I work at a adult book store, lots of inspiration ;)