The first rule of fandom is: You do not talk about fanfic.
The second rule of fandom--or of Supernatural fandom, at least--is that fandom has no rules.
Supernatural first aired on September 13, 2005, and was a show made for fanfic. It had two attractive leads, drawn into an intense co-dependent relationship in the confines of a '67 Impala, spiced up with childhood trauma and liberally sprinkled with daddy issues. Toss in a rich mythology that drew on everything from urban legends to religion, and fandom needed no encouragement to hop on board.
What we had no inkling of at the time was that this show we loved and wrote about with such passion would see what we were doing and love us right back.
It is a truth universally acknowledged that whenever there are two hot men in a TV show they must be slashed, and such was the case with Supernatural. There was one little kink, so to be speak, in the arrangement this time: the two main characters--Sam and Dean Winchester--were brothers.
The Supernatural fandom wasn't going to let a little thing like incest get in the way of our creative (and other) urges, and in fact the first fanfic posted for the fandom was a slash story featuring Sam and Dean, posted the day after the pilot episode aired.i In true fandom form, the pairing had a mashup name--Wincest--within a few weeks.
Fanfic in any fandom is strongly shaped not only by the subject matter on which it's based, but also by its more popular writers and those who run fanfic communities in the fandom's early life. While some Supernatural fan websites made it clear from the start that slash featuring the brothers was unwelcome, on LiveJournal--the early creative heart of Supernatural fandom--the popular writers wrote across a variety of genres, including slash, Gen, and even stories featuring heterosexual pairings.
Slash fiction written during the first three seasons of Supernatural was almost exclusively stories featuring the brothers Sam and Dean. Of course some fans were not comfortable writing or reading stories that centered on an incestuous relationship, and so they turned to Real Person Slash, or RPS.
Ironically, at that time RPS, which features characters based on the actors, was considered rather ethically dubious by many fans, leading to the coining of the sarcastic phrase "Supernatural--where RPF is the moral high ground."
RPS grew to be a large part of the fandom's fic for this reason, as well as the fact that there was already a well-established fandom around fanfic based on actors from The WB Network (and later the CW Network), particularly Buffy and Angel, and later Smallville, on which Jensen Ackles, the actor playing Dean in Supernatural, had also appeared.
Other fans were happy to explore Sam and Dean's relationship without sex, in Gen fic. Supernatural fandom has a high proportion of Gen fic--it has been consistently around 20-25 percent of stories posted on LiveJournal.ii It is notable that Gen fic and slash in Supernatural fandom often share the same focus on the intense emotional relationship between the main characters, which is of course at the core of the show. Back in 2007, Supernatural scriptwriter and later showrunner Sera Gamble jokingly called the show "The Epic Love Story of Sam and Dean." In fact, aside from the sex, Gen and Wincest fics can be almost indistinguishable, leading to the coining of the terms "Gencest" and also "hard Gen."
It's impossible to estimate how many tens of thousands of Supernatural fanfic stories have been written. There are over 80,000 Supernatural stories on FanFiction.Net,iii and over 43,000 on the Archive of Our Own.iv The number of stories posted on LiveJournal is impossible to estimate but a project that catalogued all the stories posted on the "Supernatural Newsletter" on LiveJournal between October 2006 and March 2010 found links to nearly 40,000 fics.v