Rarity
You still don´t quite follow how these chain of events brought you into this position. You adjust yourself a little in your chair, to make sure you have a comfy seat as you await further orders.
"So Anon," a purple pony tells you from the side, "you think you are ready for this?". You don´t know whether to laugh or take her question seriously. You could barely sleep when you first heard about her crazy plans for today. Still, you answer rather late with some quick nods. Twilight levitates her quills, a bottle of ink and papers to take notes. "I think we are ready for the leading lady in this play." She points over at a glittering curtain which... does neither move aside or anything worth mentioning.
She clears her throat, "The leading lady in this play!" she points again at the nice to look at but still rather disappointing curtain. "Rarity, that was your cue!".
After a while of silence a voice talks back. "I know Twilight, but your idea is still rather rash darling."
"Why is that?"
The white unicorn peeks through the curtain and can´t believe how she can even ask such a thing. "You want me to help you on an experiment..."
"Yeah so."
"Twilight, when I agreed to help you on one of your experiments I expected something like taking notes, be drawn, sniff on some flowers and such, but not... this!".
Twilight kept pushing "But you pretty much forced me to let you help me."
"You took care of Sweetie Belle, it´s just common decency to offer your own help when being helped in great distress. Forcing your friend to pleasure a stranger so you can watch and take notes for your own studies is just plain wrong."
Twilight looks dumbfounded, talks back "There is no right or wrong in the name of science. Unless you do something right or wrong... Then it does exist of course." Rarity doesn´t seem impressed by that argument. "Oh c´mon Rarity, it´s not like I forced you to marry a complete stranger. It´s just an absolutely natural act to verify or falsify some hypotheses I´m struggling with."
"You did bring a complete stranger into my boutique. What´s his name anyway?"
You open your mouth, ready to finally verbally partake in this crazy but still appealing sounding "experiment", "My name is M--",
"The longer you wait the longer we´re gonna stay here. Just get it over now or Sweetie Belle is gonna show up and be wondering why we tied a stallion to a chair, while you are approaching him with your backside."
Reluctantly Rarity passes through the curtains and approaches you, the stallion tied down to a chair. As much as you rather wanted to ask if you could leave and just rub one out to pictures of Rarity´s butt which you recently purchased from Featherweight(This pony may have a bright future in this metier), all your worries about this situation are blown away as Rarity sashays her big, jiggling behind with every step she does closer to you. Her look is rather shameful and directed to the floor, but you barely notice that and focus on the most clopworthy part of a pony: Her plot.
You ask Rarity if she would mind helping you blow off some pressure before you go back home. At least that would be what you wanted to say if you weren´t kinda scared to talk to one of the prettiest and sexiest ponies of all of Equestria. You are used to wank to her from a distance. The step to just non-chalantly talking to her might be a bit too much.
While you were thinking through all that, your mouth already had an answer for her: "I´m so gonna knock you up." You are kinda startled after you noticed how your dirty talk in your fantasies might not work that well in the real world. Rarity doesn´t seem to have heard that verbal diarrhea you just let out, or maybe she´s just damn good at hiding her disgust. Why does this thought turn you a bit on?
Rarity closes the distance between you while you grew ever harder and bigger with every step she took. She glances down at your rock hard prick and mumbles, "Already? I did not even do anything. Why am I complaining, at least that means it´ll be over much sooner." She watches you with one eye, expecting you to frown at that rather destroying comment on your endurance, but when the mare of your dreams is gonna get down on your pole, you highly doubt anyone would care if it goes on for a few minutes longer.
Rarity gets on her hindquarters and rests her hooves on your lap. "Oh boy..." She opens her mouth and lunges forward, she stops in mid-air, her head being surrounded by a purple glow. Also your penis is kept at distance from her face. You groan, rather in disappointment than any trait of pleasure.
"Wy dyd you stahp?" asks the fashion diva while being unable to shut her mouth.
"Yeah," you finally talk "why did you stop her?".
Twilight tells you two in a calm voice. Kinda like a grampa would tell his grandkids a bed story. "The problem is, I did not want to test the effects of a mouth on a penis. Pinkie already helped me with that one. I want to see the effects of the posterior on a member."
"Why?"
"it is an organ which does not affect the act of procreation anyhow, so I wanna learn it´s secrets."
"Why r u szo faxated on that?"
"Science," she blushes a little but hides her head behind her papers quickly "that´s all."
Rarity´s head gets released. So does yours. The new gained mobility with that thing is less than breathtaking to say the least. Rarity "Have you ever considered how hooves are also not necessary during sex but do giv--"
"I don´t care, just do it already!" You two look rather surprised and shocked at Twilight, than at each other. Twilight excuses her "I´m kinda keen on these notes, so would you guys please get it on? I mean... Yeah just that."
Rarity seems to have figured out why her magic friend behaves like that. Her awkward attempt to get closer to sex without doing it herself delights her and makes her giggle a little. She throws you a welcoming smile. You still wondered what scientific reason there could be to doing anal with your mare of your dreams, but you are more than amazed when Rarity turns her marshmallow colored plump butt towards you.
You gasp at the look of that plot. You heard tales about butts. Some were big and chubby, others were rather firm and small, some were muscular and able to clutch at will. You don´t know how to categorize this one since it seems to have the best of all. It´s a big and bouncy one, but this couldn´t just be a fatty rear, that plot jiggled teasingly inches from your face. You don´t know what kind of magic is working here, but this must be witchcraft. That ass was alive.
The unicorn bounces her backside back and forth, making her heavy cheeks jiggle a few inches away from you. Winking between them, Rarity´s puckered anal star and juicy, dripping pussy seem to beckon for your blood engorged member, but you can´t much other than just watch for now, enjoying the sight of such a gorgeous ass jiggling and shaking for you. You don´t usually get such ideas, but now you want to chew off your arms to get out of this chair and start humping that plot. Your plans don´t seem to be well thought out though.
Rarity swivels her shaking derriere around, slapping her pale ass against you. The surprising impact pushes you down into the lawn chair you try to escape, teasing you from the brief skin-to-crotch contact as she takes complete charge of the situation. Your equine member stands out from your loins, pulsing hard, just aching for another touch of Rarity´s bottom.
The confident mare continues her gyrations, knowing all too well the effect she´s having on you. "No, not yet," she commands as her sweat-dripping butt sways dangerously close to your lengthy, squirming shaft. Rarity raises her marvelous backside higher, bringing it up in front of your face. She wiggles slowly before spreading her legs wide, allowing you to see her glistening vagina and tightly clenched pucker peaking past her milky cheeks. "So wet," she moans before resuming her shaking, "So hot... Am I making you hot? Is your cock twitching for me yet? Aching and leaking and dribbling a mess all over itself, eh?"
Her sudden outburst of dirty talk fazes not only you but especially Twilight. As the purple pony watches you merely nod while catching a whiff of her musky pussy´s aroma, Rarity asks eagerly observant student with a smirk "Don´t you intend to take notes darling?"
"Wha-- Of...of course I´m taking notes. Lots and lots of notes." She says that while quickly drawing some circles on her notepad "Just don´t get too distracted. I want you to be natural and not even notice me"
Rarity slyly smiles at that reply and takes a look back at you. "You enjoying yourself back there?" You don´t reply verbally. It´s not like you could give a senseful reply anyway, with her body mere inches in front of your face, shaking so perfectly and so closely that an occasional droplet catches on your nose. Your hoof clamp around the chair(As good as possible) with an iron grip as you fight not to rise and rape her tender, teasing asshole on the spot. Your Pringles™ can of a cock is trickling and dribbling, bouncing with your heartbeats as your legs shiver and shake with denied lusts. You NEED to do something... to fuck, thrust, or just fap... anything - your dripping shaft can´t take it anymore.
Drawn up by your dick, you let go of your grip on the nicely fabricated chair and start to rise into Rarity. She expertly drops her bouncy butt into your chest. It knocks you back down into place while she scolds, "I´m in charge here sweetie. Do you want me to help you cum or not?" You nod to her, feeling a little more in control after having your urges interrupted by the object of your desire. Rarity looks back at you with lidded eyes and whispers, "Not bad. Relax, Lady Rarity will take care of herself and you."
She does not know why I exactly, but Rarity´s suddenly stricter, more dominant tone intrigues her. Twilight takes a look at her current notes and can´t find anything but sketches of dripping manhoods and a dominatrix Rarity surrounded by them. Confused by her great drawing skills, she realizes that she can´t just let these two keep all the fun to themselves. That´s when she remembered Shining Armor´s words about how special and magical her precious slit can be. Not wasting anymore time Twilight runs her hooves down to her thighs and feels that her pussy drools out of sheer excitement already.
The curvaceous pony girl´s stance shifts slightly, her teasing, shaking dance coming to a stop for just a moment as she adjusts to rest her rump against your anticipating, squirming shaft. She arches her body and slides up and down your lengthy, girthy shaft, gradually increasing the weight she´s pressing on it with every stroke. Your dripping member slips into Rarity´s tightly compressed ass-cleavage with a wet sounding *POP*. Her derriere is so hot... so sweat-slicked... so... so... perfectly tight. You aren´t even fucking her yet, though with your dick completely devoured by her big butt-cheeks, you might as well be. Yet, you know without a doubt that there would be no shame in cumming right now and turning the purple haired´s tight ass crack into a cum filled canyon.
"Oh my, does my bum feel so staggeringly that your fat, delectable cock will squirt all over me before you even get to feel the best part?" teases Rarity as she flexes one side and then the other, forcing your squirming shaft to bend in response to the two dueling masses of her booty cheeks. She keeps teasing you as she caresses your member with her hefty butt. "You have such a naughty look on your face, drooling on yourself from just a little bit of play with Lady Rarity. No, don´t stop, I want to watch the ecstasy spread through you while I work your nice, hot cock. My asshole wants to kiss it... Can I kiss your dick with my asshole?" She doesn´t even let you answer. Her back-and-forth flexing abruptly halts as she switches to an up and down motion, hotdogging your shaft in the tight vice of her cream coloured backside.
You grab hold of the chair again with both hooves, struggling not to cum while Rarity´s ass slides over your member. It´s so hot, wet, and sensual that you´re trembling from the effort of holding still, but you manage, knowing the longer you hold back, the better the finish will be. She slowly pumps your cock in her anal valley while you whine and moan, your face taking on a more lurid, sexual expression with every passing moment. When at last you feel you can hold back no longer, you groan, "G-gonna... gonna-cum-soon!"
Rarity roughly lifts up and tuts, "No, that will not do." You whine, trembling beneath her as she denies you penetration. Then, she changes her angle and presses down. Her pillowy cheeks slide around your cock, hugging it tightly as the mare´s wrinkled pucker comes to rest atop your point. You both gasp at the sudden, electric sensation, immobilized by the new feeling for only the briefest of moments. Then, Rarity is pressing down, her anus dilating to allow your lust swollen shaft entrance to her backdoor. Her asshole is even tighter than her butt´s cleavage was moments before, and as the clenching, puckered ring works its way towards your base, you know you won´t be able to cum until it loosens a little.
The owner of the perfectly tight pucker asks, "Do I have to worry about you cumming yet? No? I didn´t think so." She wiggles her rump against your groin, grinding those sweat-soaked buns on your hips and belly for a moment hard enough to rock the chair. Your pecker tries to dribble, tries to leak, but it´s clutched so tightly that not even the smallest trickle of release can escape. Slowly at first, the pony beauty begins sliding up, dragging that ring around you with agonizing slowness and allowing the bottom most portions to feel the sensual caress of her cheeks once again. Then, once she´s just below the tip, she plunges down, faster this time, and you feel almost like your cock is being crushed from stem to stern.
Knowing there´s nothing to do now but wait for her sphincter to loosen, you slump back and watch Rarity´s hourglass shaped midriff bounce atop you. Each time her bottom smacks into you, ripples of vibration jiggle through the fat of her rump, a sight that only makes your ample, rock of a cock grow harder inside her. Rarity, now bouncing much faster, looks back and says, "Such a gobsmacking cock you have, Sir. It´s so nice to squeeze as it fills me. You keep getting harder too, don´t you? Is your dick going to pop inside me? Will it burst and spurt out all that messy seed so that you can think straight again? Or is this hefty dick going to be unable to cum and be ridden by me all day long?"
In spite of her teases, the continued stretching friction of the fuck is making her tight ring relax around you. Every bump, bounce, and squeeze seems to bring you closer than before, your backed up fluids threatening to squirt out at any moment. You know it´s coming soon, but you can´t tell when. Rarity doesn´t seem to mind your discomfort, as she is bouncing hard and fast. Her whole body tenses and shakes, and as her taunts turn into moans, you realize she´s cumming, just from riding your cock!
Her orgasm drags on and on, turning the rug below you into a cum sodden mess. Yet, once she finishes, her anal sphincter goes lax, letting your ample stallionhood finally spend its seed. Rarity groans and asks, "Oooh, did you just cum in my butt before asking if I wanted you? You are still cumming, right?" She starts bouncing up and down slowly, milking your dick while it orgasmically contracts, spraying thick strands of pent up seed deep into Rarity´s darkest recesses. The white unicorn keeps talking dirty, saying, "Yes, give it to me. Give me all that nasty cum you pleb have pent up. Let my ass slurp up your cum so you do not waste any of it. Such a messy, naughty boy."
Rarity´s words motivate you to spend every ounce, every stored up drop of seed into her hungry backdoor. Your anonhood writhes in bliss for what seems like forever, but before you know it, you´re coming back down to Earth. Rarity whispers, "Good boy," to you and stands up, ignoring the cum that drips from her shady pucker to stain your curved rectal wrecker. She helps you up, and her hoof tussles your hair affectionately. Then she turns towards a special purple pony, lying in her own juices, with her hooves down and occassionally spasming around her cunt. Rarity knows she did two ponies a great favor today and leaves you two alone, walking a little bow legged as she leaves.