It's day ohdayohdayoh in Equestria, and you're trying to come up with a plan to get Flutterstalk off your back. It would help if there was some way to get inside her house, or even her head, for just a day... Idea!
You recover a strand of Fluttershy's tail hair, left in your house from one of her previous attempts to seduce you, then pay a visit to Zecorah and ask her for a transmogrify potion. She gives you a look that you don't understand, but sells you the potion at your demand.
You drop the pink tail hair in, hold your nose, close your eyes, and take a drink. You curl up in pain as your guts slide around inside you, your bones twist and bend, and yellow wings burst messily from your back. After an hour of torture, you now resemble Fluttershy.
You stand up unsteadily and try to walk around on four huge toes, always wanting to grab onto something with the hands you don't have. You try flying instead and painfully hit a few trees. After a while, you give up and just practice walking on your way to Fluttershy's house.
You walk boldly into her house, hoping to find some blueprints of her future plans, or even her diary, when you stop in shock. You see yourself, your human body, standing next to the drained bottle of a transmogrify potion.
Fluttershy must have had the same idea you had.
(S)he stares at you in shock and seems about to faint, but then you identify yourself and explain what you did.
Flutteryou kneels before you and begs for your forgiveness. (S)he starts babbling about how she was hoping to get into your pants—heh, head—because she had to know how you thought in order to reach you. And now she understands completely; she feels no attraction for ponies at all while in your form.
She was being so awful to you and it's horrible to try to force sexual affection on someone who doesn't feel it too and she's genuinely sorry and she'll leave you alone from now on. You can see in her eyes that she really means it.
Mission. Fucking. Accomplished.
You accept her apology, and give her some soothing words as you keep looking deep into those human eyes, the eyes of a creature cruelly trapped a universe away from its real home, never to see its own kind again. You just want to hug and console and comfort and love and...
Shit, you're getting turned on. The potion must have done too good a job of turning you into Fluttershy.
Flutteryou starts to realize what you're thinking, and backs away from you. You walk forward, trying to look reassuring, saying that you only want a hug, but your huge limpid eyes tell a different story.
The sexual urge is smacking you like a sledgehammer in the loins. When these equine bodies get horny, they go full tilt, overwhelming all attempts at rational control. Ponies have lifelong experience in reining in this intense animal lust... But you are losing the battle.
Flutteryou yells in panic and tries to run. His/her resistance only makes you wetter. In a bound you're on top of him/her, your equine genitals winking in a horse code of eager need.
Flutteryou screams no, and some deep part of you realizes that s/he really means it, but you're too far gone to care.
Blackout.
You wake up in Fluttershy's bed, Twilight lecturing you in sorrow and anger as Dash and AJ grimly hold you down. Pinkie and Rarity are caring for an unconscious Flutteryou, trying to clean up gallons of lustslime from the ravaged body.
As you try to collect your thoughts, the potion wears off and you change back to your own body. It's a lot quicker this time, but it hurts even worse. Twilight halts in mid speech and starts to stammer.
"Anon...? But if you're Anon, then that means... Oh, Celestia, NO!"
Everypony shrinks back and stares at you in disgust and horror.
You try to explain but it just makes things worse.
You slink home.
You just raped Fluttershy, and thereby also raped yourself... and it doesn't end there.
When Fluttershy recovers, all she'll remember is that when the situation was reversed, you lost control and gave in to your desires. Now you'll never stop her from trying to take you.
You've lost any trust and sympathy that you had from her friends.
You've fucked yourself over for the rest of your miserable life.
Fucking you.