Going on an inter-dimensional date with her marefri—sorry, girlfriend—was not supposed to lead to the existential crisis of a lifetime, but there was Rarity anyway.
Rarity found she had a secret suitor from Pinkie. In her attempt to find out more a series of events lead her to bring her suitor home! A place she never takes anyone.
Applejack and Rainbow Dash ended up together in the main universe - through the portal, it was Applejack and Rarity. It's a fair question who made the right choice - from the sidelines, the other Rainbow and Rarity wonder if it was them.
Pretending to be Rarity's boyfriend at her cousin's wedding isn't the weirdest thing Sunset Shimmer has ever had to do for her friends. It might even be fun. But just what is Rarity hoping to get out of this subterfuge in the first place?