Fluttershy doesn't think much of it when Pinkie Pie asks her and her marefriend, Rainbow Dash, to go on a double date. They have been dating for a while, and so have Pinkie and Twilight, so she is looking forward to a nice day out with her friends where, surely, nothing out of the ordinary is going to happen. Nothing at all.
Pinkie Pie's intentions are entirely the contrary.
Contains: polyamory, lesbian horse smooching, sex is vaguely implied in conversation