Sex Pollen is fanon having its origins in a common Science Fiction/Fantasy plot device: characters encounter a rare/alien/magical plant and spores/pollen from it cause them to behave in ways they ordinarily would not. Often in commercially produced mass media this plot device is used to produce a comedy story, or a cautionary tale wherein various ethical/moral issues are explored, or both.
Sex Pollen as a plot device is most commonly found in DC Comics fan fiction, where it is often used as a plot device to get two (or more) characters into a situation where they must have sex. Sex Pollen as a plot device is by no means limited to DC Comics fan fiction (Star Trek, Stargate SG-1, and Stargate Atlantis spring to mind), but it's very common as a theme in DC Comics fan fiction, due to the fact that in canon, one of Batman's "Rogues Gallery", Poison Ivy, has repeatedly used various psychotropic plants (spores and/or pollen) and her own pheromones to seduce and mind control others. Her obsession with Batman has lead her to attempt to seduce him on numerous occasions. Other DC Comics (Legion of Superheroes, Teen Titans, various Green Lantern titles) have also featured stories (from the 1950s's to date) wherein the spores or pollen of alien plants have caused various "behavioral problems" in characters.
Its introduction into Supernatural fan fiction/fanon probably comes by way of several prominent (and prolific) DC Comics fen entering Supernatural fandom.
Whatever the fandom, Sex Pollen is a mainstay of Crack fic.