This is what results after reading through some metric fuckton of pastebins in search of Sun-butt stories. It is a comedy but you horny buggers will take it as clop anyway because it is a sexy comedy. Yes.

I refuse to apologize.

>In the six months since you first arrived in Equestria, you have learnt two very important things.
>The first is that ponies are cute. Like, really, utterly, every-possible-definition-of-the-word cute.
>They're plain old nerdy cute, happy cute, coyish cute, small-child cute, old-people cute, innocent cute, grunge cute.
>And, yes, sexy-cute.
>Like, really sexy-cute in a few cases.
>Horses know how to hint all sorts of things with their eyes. 
>Which, you guess, is kinda fortunate, seeing as every other race on this literally godforsaken planet treats charisma as a dump stat.
>Fuck you, griffons. You know what you did.
>So. Ponies. 
>Would you? 
>Answer, yes. 
>Of course, yes. They are that goddamn cute that anyone, worn down over enough time, would say yes.
>You just want to see the little expressions on their faces as they orgasm. Imagining the scrunch and girlish squeal is enough to make the d go D. 
>But there's one problem with that. One large, important, very, very crucial problem with that.
>Could you?
>Hell to the F no.
>They are bare back there. Only a hole hidden somewhere in their tails that poop and piss flows out of; nothing you can really fit inside.
>They reproduce by budding (which makes a pregnant mare horrifying in and of itself) and so there is, to an extent, a sense of romance.
>And it is cute. Pre-school cute. All blushing and held hooves and shit.
>FYL.
>The closest they come to sex is kissing. And that's like, chaste kissing. Tongues stay within mouths, press lips together and hold. Try to make any lip movements and prepare to be treated like you'd just mounted your grandmother.
>You are Anon in Genitalia-less Equestria.
>There is no hope.

>It's early Spring in Canterlot, and the temperature's rising. All morning, the air's felt lighter, cheerier. The Winter's cold wrapped up and the Summer's heat unfurling. 
>The windows to the Castle's libraries have, after a long Winter kept breeding dust, been let open, and the fresh air is wafting in.
>It's perfect. Heaven. You take in deep, gulping breaths of it then sigh it out with satisfaction.
>This annoys Twilight to no end. You can tell by the way she glares at you and mutters about how it annoys her to no end. 
>But that's alright. This is your lunch-break. It's her fault if she wants to spend it being nerdy.
>"You could just, y'know, leave," Purple-Smart mutters. 
>You give her suggestion due consideration.
"Really? I thought I wasn't allowed out of your sight."
>"For your own protection." Twilight glares at you, her eyes all lined with red veins like ivy. "And I'm coming round to the opinion that you're in more danger if you stay here."
>Briefly weighing up the option of staying coped up in a library, waiting for Spike to come back with lunch before continuing to browse it for obscure, unicorn texts, and freedom, you act like a Scot and say No Thank You.
>Joke. You are totes out of here.
>"Make sure you're back by one," Twilight whispers as you stand up, brush your suit off and stroll with great determination towards the door. You throw her a little salute back before banging open the doors and walking into the sunshine.
>Man, this is the stuff. You just stand there for a moment, arms held wide, and bask in the feel of it.
>The warm, hugging tingle as it rushes over you; the glorious sense of heat and love from a burning nuclear furnace. 
>Really, it's weird how people ever managed to stop worshiping the sun.
>You open your eyes and grin up at the castle before you, its golden domes glimmering white in the light of your reflected deity.
>Lets go explore god's house.

>Getting in was a lot easier than you thought. A few nods from the Royal Guard and you're in there, idly wandering the places the plebians cannot.
>You sure hope Equestria's short of masters of disguise, 'cause this system is so prone to abuse.
>Ah, well. Not your problem.
>You admire the artworks hung everywhere as you take a dally through the castle.
>My, that is a nice picture of Princess Celestia beating some griffons. Really shows off the abject terror in their faces.
>Or this statue of a rearing Sun-horse one-hoof-kicking a minotaur riding a dragon in the face whilst her horn proudly skewers three Diamond Dogs in the chest. 
>And oh, oh just look at that fresco of ponies standing on a globe and marveling at the rising sun, with the moon hanging limply on the other hemisphere, all alone and by itself.
>And so you absorb the culture. The history. The general egotism and narcissism that is the calling card of history.
>It's a good thing you've met the princesses. Otherwise, you'd think they were a bit full of themselves.
>Ponies pass you by occasionally as you wander. Maids carrying wash-buckets and mops on their backs; courtiers with rolls of parchment and important expressions; Guards who smile and happily point out where the most important areas of the castle are.
>And then, to your surprise, a princess herself.
>It's Celestia, the big white one. Top dog in the Equestrian kennels. You hang back a bit out of respect and apprehension; it turned one ten minutes ago, and for all you know, Twilight's informed her.
>But your eyes have been trained by all the culture you've absorbed, and they're not going to let such beauty walk by uncritiqued.
>Unf
>And stuff
>Or maybe

Could I describe the way she walks
In words that do not fail
To capture, without slip or balk, 
The motion of her tail

The way it sways and curls and slinks
Like streams of dawn-cut light
And hints at something soft and pink
That hides just out of sight

>Wait, what?

>You put the poetry on hold and check out the Sun-butt
>Yes, your eyes doth not deceive. There is something in between the white, fleshy mounds, revealed every so often as her tail glides softly out of the way.
>Has she sat on some cotton candy?
>Or a cake?
>You have heard such tales from Twilight.
>And now you're wracked with indecision. Should you mention this to her? Maybe this is a thing Royalty do, and to bring it up would be crass.
>Okay, no. That's dumb. Royalty are just like everyone else. They do not intend to sit on cakes.
>You hope.
>You groan and hate yourself, in particular the little moral core that's operating your brain right now.
>Plus, you know Celestia. She is cool. You'd probably call her a friend, and hells, you ain't one to let a friend go out with cake stuck to their butt.
>Time to go and get back to being bored in a library under purple unicorn Hitler herself.
"Erm, Princess? Princess Celestia?! I've, um, got something to tell you!"
>"Hmm?" Sun-horse stops and looks back, and damn it, she looks cute. Sexy-cute, not school-child cute. This is full horsey-woman on display.
>"Ah, hello there, Anonymous. I wasn't aware that you'd be in the castle today. How lovely it is to see you. I suppose Twilight's sent you here to see me about something."
"Ah ha ha, no not quite."
>"Ah. I see. Gave her the slip, did you?" She winks, a cheeky grin on her face. "I won't tell if you won't."
>Stop it woman. Wait, no, don't stop. Well, carry on being a based horse but cease being sexy-cute. No relief is coming until you're safely back with your fap stash.
>"Well, anyway, what was it you wanted to talk to me about?"
"Oh. Well." You reach up and rub at the back of your neck, looking for the words. "Well, you seem to have something on your..."
>"Hmm?" She steps a little closer, ears pricking and turning towards you. "What was that?"

>You make a few encouraging rubs at your backside, eyebrows waggling.
"You've got a little something on your..."
>More backside rubbing. More eyebrow waggling. Her eyes flare as she gets the hint.
>"Oh! Oh, I see. Thank you for bringing that to my attention." Her horn flashes once, and a cloth appears mid-air, then gives her buttocks a quick brush. "Did I get it?"
"Hrm," You throw a quick glance behind her. "No. Still there. Sorry."
>The cloth returns and rubs a bit more thoroughly. The above scenario repeats itself near exactly. 
>"Hmm. Hold on." Another flash of light, and this time a hand, or rather, a hoof-mirror appears. She angles it behind her, then smiles, softly snorting. "Ah. I see."
>"There's nothing to worry about, Anonymous. Whilst I appreciate your concern, I'm afraid I'm more than used to that particular blemish. It's a mark both I and my sister share, a consequence of the nature of our manes."
>There's a sneaking suspicion gathering in the back of your head, two parts disbelief to one part desperate hope.
"Wait. You mean that poofy pink thing between your legs is?"
>"Yes. It is, ahem, the dual holes of my defecation."
>Oh.
>And you were actually hopeful for a few moments there.
>"As both my sister and I have tails that are more insubstantial than any of our subjects, we cannot defecate through them as do other ponies. And so, we were instead born with these... extra parts, so to speak."
>She turns around. Her tail flicks up. Your heart stops.
>That is most definitely a vagina and a ponut.
>You have located the Holy Grail, and Mister Jones, there's only one choice here to take.

>"They are incredibly ticklish, so I would prefer it if you kept your investigations solely visual." Celestia sighs as you stand there, tears glistening in your eyes, palms sweaty and legs shaky. "Disgusting, is it not? I've grown used to it over the years, but I can understand your distaste. This, I feel, more than anything else, is what sets us alicorns apart from the rest of pony-kind, this horrible mar on the Equestrian form."
"It's the most beautiful thing I've ever seen," you croak. Celestia steps back, disbelief sketched by her wide eyes and cocked eyebrows. "Marry me now, please.・
>The Princess caries on giving you the wide eyed treatment, before defaulting back to her lazy smile.
>"Anonymous, please. I understand that you mean well, but I speak truly when I say that this has been a source of much discomfort to me over the years. It is a rather sensitive topic, and I would appreciate if you do not joke about it with me."
"No! No, Princess! You misunderstand." You fall to your knees in front of her, beseeching with both hands. "I know what this is, and it's not a blemish, not a mar! I have one too!"
>"You ... You do?!" You rethink that statement.
"Okay, well, no, not one of those, well, maybe half of it, but it's like, I've got the counterpart. The key to your lock, the handle to your broom, the socket to your plug, the bow to your viola, and baby, trust me when I say we can make such beautiful music together."
>"Anonymous..."
"Please! Just trust me. I'm your friend, right? Believe me when I say I know what that is, and you are wrong. It is not ugly, it is a gift."
>She still looks disbelieving, so you pull yourself back up and put on your best tradesman smile.
"And I'll prove it to you. Give me twenty minutes and I will make you glad you've got one of those."
>Celestia eyes you carefully. Cannily. With a thousand years of experience. 
>"Five minutes."
"Ten."
>"Five."

"Fine, five." You make angry hand gestures then stow it. "And I'll have you know it's unusual for the lady to be talking the gent down on time. But it doesn't matter. I've got enough time. I can do this." You give her two thumbs up and a winning smile. "Where's the nearest bedroom?"
>"Bedroom?" Celestia sighs, closes her eyes and shakes her head. "Anonymous, I don't have time to go to a bedroom. Or, to be frank, to take part in these games of yours."
"Okay. Okay. No bedroom, then. We'll do it out here. In the open. Where everyone can see us."
>You rub your hands together to get some warmth into them, toss off your jacket and crick your neck.
"I guess they won't care because nobody but me seems to know what sex is. Turn around, Princess. My five minutes starts now."
>Eyes rolling, Celestia complies. Body quivering with anticipation, you lower your hands down to Celestia's butt cheeks.
>And make contact.
>It's everything you've ever imagined, everything you've ever dreamed of. Softer than the fluffiest pillow with a firm underlay of muscle beneath; plump doughy-ness above tight substance.
>Perfection in a butt.
>You spend a while just kneading it, carefully pulling and pressing her body back and forth in a rustling of coat, skin and flesh. Her sun cutie-mark rolls beneath your fingers, white and yellow switching beneath your digits.
>"This is a massage," Celestia says, one eye visible as she looks over her shoulder at you. "And a very nice massage at that, your hands seem to be well equipped for it, but not something I haven't encountered before."
>You smirk in response, letting your thumbs slide slowly down the crack of her ass as your hands move south.
"That's just the warm up, Princess. We ain't even started."

>You lean down and scooch closer to the royal buttocks, hands peeling back the rolling cushions of her cheeks to reveal the beautiful slit of pink within.
>And you don't care if it looks like a horse's. Man, you'd be down with anything at this point.
>You puff a breath of hot air at it, and smile as Celestia shivers.
>"Anon, I thought I told you. I'm very ticklish back there."
"You're also mine, which means we do what I want to, for the next five minutes."
>"Four."
>You pull a face. Time to make her stop thinking about the time. 
>Lips first: gentle kisses round the landing area. Small suction on the soft, hairless flesh of her bum ・pull parts of her up with your lips before releasing. Gentle nips that make her tense, then shudder.
>And the hands kept in motion, constant motion. Sliding round and round, adjusting thighs and widening legs, pulling the dock and tail up until they stand ridged by themselves.
>Groove round her rump, a bulge of fat proceeding. Reach down and make sure your dick doesn't snap itself in two. Toy with the vagina by pulling the flesh around it so it stand taut, open and vulnerable.
>Then, in a trial of tongue daps, you lick the hot-pink slit itself.
>Celestia's breathing rate, already unsteadied, hitches, and she leans back into you, unconsciously pressing herself into your face.
>"Anon, I'm sorry, that... I don't know what came over me, but-"
>Her babbling cuts off as your hands grip her thighs and pull her firmly back onto your face.
>She's not getting away that easily. 
>You continue, keeping it gentle, slow. Like you've got all the time in the world rather than three or maybe two minutes left. 
>A tongue slipped between her folds, pushing the opening wider. Smoothing the outside with your lips, greasing it in saliva and flowing down.

>There should be a clitoris here somewhere, right? The magic button, the secret access panel. You launch a finger into her depths in search of it, smiling when you hear her breath break and feel wetness where you tongue could never reach.
>"A-AH!"
>You pause. You try it again, gently rubbing at the soft nub within. 
>"O-Oh, ohhhhh~!"
>Alright. Clitoris located. Also, it seems the Princess is a screamer. You toy with it a few more times to confirm, making Celestia squeal and gasp like the world's greatest sounding instrument.
>"Anon, I think I'm going t-to pee!"
>Her backside thrusts firmly into your face, wedging your nose and mouth tightly into her half-liquid pussy. And then the tidal wave comes (hue) in a flow of yellow water.
>You pause, frozen as Celestia pants, her body shaking against yours with each breath. 
>This is not pee. Pee, you think, neither smells like this nor tastes this rank.
>Female ejaculation is apparently a thing for pony girls. You make a note of this in your head right next to "Don't stand in the way of an oncoming train," and "Sharks get testy when being stalked."
>But back to the action: she's got hers, so it's time for you to get yours. You wipe her cum of your mouth and stand, unbuckling your trousers.
"So. You ready for the main course?"
>A panting pony princess slowly emerges, head rising past her shoulders. Her eyes are at half-mast, her mane's in disarray and her crown is crooked upon her head.
>"There's more? But it's, it's been seven minutes and court..."
>You revolve a hand slowly round her back, dragging your fingernails up and down.
"Well, I guess we could call it off. I mean, I wouldn't want to leave you without a full taste of what I can offer, but if you really want to go and sit in a stuffy old court, well, who am I to stop Royalty?"
>She's smirking at you now, mouth still open and pink tongue flicking round her lips.
>"Oh, just do it, Anonymous." Her eyes close and she smiles, half-serenely. "I'm too old to play these sorts of games."

>And so you let the pants fall down.
>Boner is released. We are all fucked.
>You tap it a few times against Celestia's ass, enjoying the way it makes her butt ripple, before lining it up with the pussy and stroking slowly up and down, mingling your pre-cum with her girl-cum.
>"What is that?"
>You look up to find Celestia trying to get a peak, neck craning this way and that as she tries to spot what you're rubbing against her privates.
"Oh, this? This is my pussy."
>You reconsider those words.
"Well, this is my equivalent of a pussy. It is a dick. Penis. Willy. Manhood. Length. There's a lot of euphemisms."
>"A lot of what?"
>You try and think it through before realizing you don't much care.
"Never mind. I'll tell you later. The important thing is that it can do this."
>And you enter her.
>She is tight, wet and eager to please. Her body is soft velvet brought to life and hungry, squeezing and sucking and trembling as you slide ere deeper.
>Celestia herself, meanwhile, has the most spaced out look on her face you've ever seen, her eyes staring far out and her mouth constantly stuck to pant. The little twinkles in her mane start flashing like crazy, as if each one is supernova-ing out of sheer surprise.
>Then you hilt, and are happy to just stay there for a few seconds, pubes scrunched up against her vaginal lips whilst the pussy itself gives your penis the greatest welcome a dick ever received. 
>Poor pussy. It must have been a hard thousand years, being attached to someone who didn't understand it. It's alright, though. You're here now.
>Gently, you pull back out, skin catching and tugged forwards in a blissful surge of pleasure. You exit seems to take the air with it: Celestia groans out one long, unbroken syllable, her voice a song as of yet unheard in this world.
>And then back in, a little harder. The note rises, timbre strengthening and pitch soaring.
"Faster?"
>"y-yes."

>You oblige, thrusting in and out at medium pace. The walls echo with Celestia's moans and wails ・morphing into a carnal symphony played from one instrument. You hilt and grind, reverberating one long scream of pleasure.
>"F-FaSTer!"
>More strength put into each slap of flash, more power into each drive. Her hooves slip and slide on the marble floor before her front buckles and falls, crown at long last tinkling off her head and rolling far away.
>You pause to make sure she's alright, but milliseconds later, and she's screaming:
>"FASTER!"
>Harder. Longer. Stronger. You slam into the Princess again and again, driving her inch by slow inch across the floor with your ferocity. 
>The sound of doors slamming in the distance, of ponies rushing towards you, begins to mar the sound of your lovemaking, but Celestia's horn flashes yellow and a bubble stretches out round you to fully encase the room, and the sound of them banging at the doors turns into a percussion beat that's only slightly off-tempo to the thwhack of your crotch meeting her rear.
>There's a flash of purple.
>"Princess!"
>A wild Twilight has appeared.
>Please fuck-off, Twilight.
>"What are you doing to the Princess?"
>You begin to respond, stopping the beat briefly, but Celestia kick starts it with a slam of her butt before lifting her head off the floor and meeting Purple-Smart head on.
>Twilight steps back. You can only imagine what sort of crazed Celestia looks like right now.
>"T-T-TWIIIlight Sparkle, my MOOOST faithful pupil~ If you a-ttt-empt to stop Anonymouuuus~ from carrying out his duty I will・WIhiill,"
>She pauses and just grunts for a while. Twilight gives you a look bred from bafflement and worry; you give her a happy shrug as a thank you present back.
>"I will do bad things to you. Bahahad things. You will not enjoy them. Mmmmmm~ No."

>Her snatch grabs hold of your dick suddenly, the soft, pliant muscles turning unyieldingly solid. But in a nice, comforting way.
>And then she's cumming, liquids flowing round your thighs and trickling down as her butt starts slowly rotating against yours, rubbing and jerking in a tight little circle.
>Oh, man. You are not going to orgasm with Twilight watching you. That's too weird. It is not a thing you are going to do. Not.
>And here it comes: You fire, shot after shot drawn out of you by her inwards undulations and the motion of her hips.
>The door's burst open as Celestia's magic fails and guards and maids and courtiers alike pour into the room. 
>Amidst the confused shouting, you sink down, still-solid dick plopping out of Celestia's cunt and dripping fluids onto the floor. You lean across her still raised butt and give her back a solid pat.
"See? Vaginas are awesome, Princess. Not things to be ashamed of. We should do this again sometime."
>She mumbles out a "Mmm." in response, eyes tight closed and tongue dangling out onto the floor.
>And yes, it is very, very cute. Très sexy-cute.
>You stand on shaky legs and wobble over to where Twilight is busy lecturing the crowd.
>"And so it's clear to me that Anon and the Princess just had a mutual loss of bladder control. I understand that this must be very embarrassing for both of them, which is why the Princess has fainted, but we should be encouraging to her in her time of need, and, more importantly, discrete."
"Nope. Not what happened at all," you interrupt, coming to a stop by Twilight. Your naked body shivers in the breeze.
>"Then what exactly were the two of you doing?" Purple Horse is incredulous, in a very cute way. You resist to pinch her little cheeks. "I mean, the two of you are covered in fluids that came from... those things, so you clearly must have been peeing."

"Nope. Not peeing. Sex. It's a thing a man and a woman do together. Lots of fun."
>One purple eyebrow rises. "Sex? That's not even a word, Anon. There's no need to feel so embarrassed: you're amongst friends here."
>The whole pony crowd nods at you. You groan and try to find patience.
"Look, okay, it's a thing. You can't do it; we can. It's not peeing and we didn't wet ourselves."
>"Prove it." Twilight looks smug, a little look in her eyes that says "I've got you." "Why don't you have sex with me right now, huh? Bet you can't."
>They don't know what sex is, you remind yourself. They see nothing wrong with just suggesting it like that.
"Well, I can't because・well, ya see-"
>"We have arrived!"
>The world flashes blue and glass starts breaking. Hands on ears you turn around to find a Princess Luna arrived on the scene.
>"Sister! We heard thy screeching and have arrived to slay the culprit," she bellows at a still happily drooling Celestia. "Why is thy visage one of pleasure? Death is not an enjoyable act, dear sister! Try to look a little frightened!"
>You remember Celestia mentioning how Luna also had a "blemish"
>They don't know what sex is, you remind yourself. They see nothing wrong with just suggesting it like that.
"Her." You point at Moon-butt. "I can demonstrate what sex is on her."
>"Pray tell, what doth thee refer to?" Luna is strolling up to you, still yelling. 
>Man, you wonder what other noises she can make. 
"Hey. Princess Luna. You want to feel as happy as your sister, Princess Celestia, is looking right now?"
>Luna looks round, before turning back to you.
>"We are intrigued. Proceed."
>You smile, and crack your knuckles.
>Today was a "Finally got some" type day.