Rarity loves rimming.
Moondancer smells her own armpits, and she scratch and sniff her pubes.
Applejack has helped Sunset to have sex with real horses since horse cock is one of the very few things she does miss from Equestria.
Fluttershy loves helping animals "in need" when no one's seeing to please both them and herself.
The CMCs have played with each other inside their treehouse since they genuinely believed playing like adults was going to help them get a cutie mark.
Marble Pie is actually a guy.
The Apple family have traditionally been far more incestuous than the Pears.
Starlight is probably into diapers or some kind of weird age regression play.
Pinkie Pie is way too innocent and nice to be the sex expert mare people make her to be, and if you were to pull your cock in front of her she would probably not understand and be weirded out by it.
Celestia has fantasies over Discord for hundreds of years since she's the only one that truly knows what he's capable of pulling of.
Discord has raped before.