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.
"I've thrown some pretty super parties in my time, but this one must be off the charts, cause two of my besties are finally trying to hook up! Wait, four of them are! Ohmygosh, they're all trying to hook up! ...Maybe this isn't the best party ever."
Celestia likes her routine. Tax reforms, grants, laws and construction projects are all a princess needs to be content. It's just her luck that for Pinkie Pie, "content" is not nearly enough.