Honestly, I post my stories on AFF and other sites BECAUSE they're not marketable. Nobody (nobody in their right mind, anyway) is going to pay me to write PHINEAS AND FERB porn, or even the furry stories I post on Hentai Foundry and Inkbunny. If I thought anybody would buy such things, maybe I'd slap a price tag on them.
Having had a sort of "previous life" as a fanfic writer in a different fandom years ago, I just accept the fact that there is going to be a ton of slash wherever you find fanfic. Nature of the beast, and all that.