It's not her fault. Really, it isn't.
Rarity has constantly been gushing about how wonderful and attentive Pinkie is, and Fluttershy always listens dutifully, like any good friend would. Rarity really loves Pinkie, so much so that they're engaged to be married. And when her best friend constantly talks about the things she loves about her marefriend, is it really any surprise that Fluttershy begins to see those things as lovable as well?
Cow-riding assistance by Aurora.
Assistance by Karrakaz, Squinty Mudmane, and Setokaiva.
Rarity is having a rough day. Her friend Pinkie Pie comes to her rescue. Now Rarity has a problem: What do you do when your Prince Charming turns out to be a princess?
Pinkie and Rarity are in love, and engaged to be married. Fluttershy couldn't be happier. Well, unless she were Rarity.