I have news for you. Most writers who actually make a living writing are self-published. Sure, there are plenty of traditionally published writers, but a successful self-published writer can make a lot more money than being under contract for peanuts per sale.
Even many writers who have a traditional deal are now hybrid writers and self-publish a lot of their own stuff.
And yes, erotica is one of the biggest themes of books now (the e-book revolution means that no one sees the cover you are reading in public, they just see your e-reader) and that means that every genre is flooded with erotica titles.
But with 50 Shades of Grey mainstreaming “Mommy porn”, even the big five are pumping out erotica lines and titles by the bushel. So it isn’t a “self-publishing” issue, it’s a consumer issue. Our society craves weird and kinky writing.
Cover art for "50 Shades of Ghraib".