Title: The Rise of Futaquestria.

Main characters: The Mane Six, Spike, and the CMC.

Genre: Alternate Universe, Comedy, Romance(NSFW), Adventure.

Premise:
With the growing number of Mares (and lesbian couples) and the dwindling number of Stallions, Princess Celestia's top researchers start working on a formula to allow Mares with female companions to become futanari so they can have children of their own.

However, due to a freak accident the lab explodes, resulting in the formula spreading across Equestria like a strange disease, infecting anypony that comes into contact with it and transforming them against their will into sex-crazed creature's that can think of anything else but fucking. While Mares end up becoming futas when they are hit with the formula, Stallions are in fact turned into Mares (though they still keep their genitals).

Now it's up to Twilight and the remaining ponies to find a cure before it's too late!