Behold at the Futadom. Here in this wonderful world there exists a third gender, the futa. These marvelous beings were born with the body of a beautiful woman (vagina included) as well as the dick of a man, but it doesn't end there. These amazing Futa have unique cum which "Bind" men and woman to them, creating an emotional connection between them. Once bound people crave Futa cum, much like a drug, even going through withdraw when they don't have it for to long. Of course this isn't as bad as one would think, as Futa cum is very nutritious, and delicious (eventually).
There is much, much more, but that is for you to decide. In many Futadom stories those whom are bound become dumb, but you don't HAVE to go this route (though it would be the most accurate). The Futa could been there since the dawn of man, created by there futa goddess or perhaps they were created by science, or a "mistake" in nature, there goddess a fabrication to justify what they do.
Perhaps futa let men and women have there childhood, or maybe those little kids are just to cute to resist (the reddit cannon doesn't allow it, but this one will, if someones brave enough to write it). Maybe the futa are an invading race of aliens, or even...Magic Elev's? Well anythings possible, and isn't that one of the draws to this world.