Celestia and Luna married the mane six, now they have a harem and a honeymoon to deal with. In all honesty, happily ever after isn't all it's cracked up to be. At least there'll be room for orgies.
Celestia and Luna have both fallen for Twilight Sparkle. So naturally the only way to fight for her love is to make themselves normal! At least, as normal as two princesses can be.