Under An Outhouse
You follow Rarity as she makes her way towards the town's park. It's early in the morning, before anypony in town is up, except maybe the Cakes. Walking behind her gives you a peek under her tail as it swishes, not that she's ever tried to hide herself from you. Thankfully it's not too far from her boutique to where the two of you are going; when you reach your destination, the only actual building in the park, Rarity leads you behind it to a hatch. Being courteous you open the hatch for her, she thanks you and jumps in. You hop in after her and close the hatch behind you, leaving the two of you in total darkness for a moment until Rarity uses her magic to illuminate your surroundings.
You take a moment to look around as Rarity retrieves several items from her saddlebags. The room you're in is a concrete box, the walls and floor are decorated with runes, most of which look to be inactive, and above you is a singular, good sized hole. You're distracted from your observation by the sound of Rarity humming and placing things in a shelf built into one of the walls. She's brought two big jugs of water, several sex toys and a box of prepackaged snakcs, all of which she has organized in the shelf. With the items out of her saddlebags, she hangs them on a hook near the hatch.
"There, we're all set, darling, now there's nothing to do but sit and wait." Rarity said as she took a seat in the center of the room.
You join her, sitting next to her and wrapping an arm around her. She leans in and rests her head on your shoulder, the two of you are content to sit like that for a while. After a bit you sneak your hand under her and stroke her sheath, making her groan softly into your shoulder. You keep stroking her marecock, enjoying how it gets bigger with every couple of strokes, all the while Rarity moans into your shoulder. You decide to take it further, you pull her into your lap, holding her against your chest as your other hand wanders down to her marehood and your fingers slip inside of her.
As you work her marehood the two of you are showered with a deluge of piss. Thanks to the runes on the walls nopony can hear or see you two down here, but that also means you don't know when they're using the outhouse above you. The two of you certainly don't mind the hot shower of piss, which turns parts of Rarity's coat yellow. Nothing else comes after that though, likely just one of the early birds in town on their way to work; later on there will certainly be more coming and more often. You two just sit there, Rarity in your arms, and enjoy the feeling of being soaked in piss.
You decide to pass the time in a bit more interesting way, and kiss Rarity on the lips. She leans back into you and returns the kiss, the two of you making out for a couple minutes. As you two break the kiss a ball of shit falls from the hole above you, splattering on Rarity's head, and you lean forward just in time for a second one to land on your own head. This is what you two had been waiting for, as a third, loose one, splattered down on both your faces. Your cock throbbed at that, pressing against Rare's ass cheeks as you took some of the shit and started rubbing it into her coat.
As you work the shit into her coat more comes down from above you, Rarity turns around and pushes you onto your back. "There's probably going to be more shortly, so let's let it land on us for a bit, darling."
You nod and the two of you snuggle closer while laying down on the floor of the outhouse pit. More came splattering down not long after that, along with a volley of piss that you opened your mouth to catch some of. Rarity explained that there was an event planned for today, a food festival of sorts and that there would be no shortage of waste raining down on the two of you. That certainly proved true as several ponies added to the shit pit over the next couple of hours, however it was still early and the event was probably just starting. That didn't stop the two of you though, and after a couple of hours in there, Rarity needed to relieve herself as well.
She wasted no time in standing up, trotting around so her rear was over your face, and squatting down. Her shit plopped out in neat balls that stuck to your forehead and face, while another pony's poop fell from the hole above you, landing on your hard cock and belly. After depositing the last of her shit on your face, Rarity stepped back around and climbed on to you. Wasting no time, you grab some of her shit with each hand, rubbing more of it into her coat, and massaging her a bit as you do. She moans as you work the shit into her coat, staining it brown as you go. As you work the shit into her coat with one hand, you reach down and aim your cock with your other; after a minute or so you've softened enough to unleash a powerful stream of piss right onto Rarity's marehood and flanks. The filthy mare moans as you recolor her ass to a light yellow and uses her magic to smear some shit on your chest.
Once your bladder is empty, you stroke yourself until you're hard and then pull Rarity's hips to yours and press your cock between her asscheeks. She groans as she feels your filthy cock pressing against her butt, her own cock twitches and throbs against your belly, so you take this opportunity to lean up and kiss Rarity on the lips. Both your hands migrate to her stained flanks, working your hips and her's, you line yourself up so that you can fuck her. Your filth slicked cock easily slides into her marehood making her moan into your mouth. The two of you break the kiss as you start properly thrusting, your cock sliding in and out with ease as you both moan. As you fuck another pony drops a load of shit onto the two of you, the shit lands right on Rarity's butt and splatters on both your hips, and it only drives your on. You thrust harder, your hips slapping her ass as you repeatedly plunge your cock into her tight marehood. Her own cock slaps against your belly with each thrust, coating parts of it in shit.
You keep up your pace for a couple of minutes until you're feeling close; in one fluid motion you roll over so Rarity is under you, you pull out and spin her around before pressing your filthy cock against her lips. Rarity opens her mouth and you waste no time slipping your cock in, and fucking her face. You quickly hit your peak and blow your load down her throat, keeping yourself hilted in her mouth until you've emptied your balls. Once you've finished, you pull out and spin her around again so you can take her own cock into your mouth. You begin bobbing your head up and down on her delicious shaft, the shit coating part of it only added to the experience of sucking her off. Rarity didn't last long though, after a minute or so she used her magic to push your head down to the base of her cock as her flare expanded and she pumped her cum down your throat.
When her balls were empty and she let you go, you pulled back gasping for breath. Both of you were a little tired, but still not completely satisfied. The two of you moved to a corner to take a break, having some water and a proper snack as you two cuddled together while watching the pile of shit and piss in the middle of the room grow. The event was in full swing and ponies were using the outhouse more. It would likely be another hour or so before ponies were constantly using it though, so you and Rarity decided to wait.
Ponies add to the pile of shit on a more and more frequent basis over the next two hours as the two of you wait. Soon it's clearly at the point where ponies are using it back to back, and the two of you are ready for more fun. You stand up with Rarity in your arms and toss her onto the now much larger pile of shit, the piss has long since covered the whole floor. Rarity yelps as you toss her in and she uses her magic to drag you with her. The two of you are both mostly covered in shit and piss, and more keeps coming down from above. What parts of you aren't covered in shit are being fixed by Rarity's magic, while you work to turn her entire coat brown. Quickly you're both completely covered in shit, and more keeps coming down from above.
You move so that you're right under the outhouse seat, tilt your head back and open your mouth; more shit drops down, this time some of it lands in your mouth, so you pull Rarity into your arms and share a shitty kiss. The two of you pass bits of shit back and forth as piss rains down on you from somepony above. Your cock is hard and pressing up against hers as you two continue to kiss, and roll around in the muck. Eventually Rarity rolls off of you and rolls you over onto your hands and knees; after walking around behind you, she mounts you like a stallion and pushes her shit covered cock into your ass, making both of you moan.
Doesn't hesitate to start fucking you, driving her cock into your ass over and over again, her hips slapping against yours with a wet squish. As she's fucking you her magic pushes your face into the pile of shit, and she commands you to eat; you take a good mouth full of the mix and start chewing as she picks up her pace. She keeps pounding your ass for another few minutes before pulling out, using her magic to spin you around and stuffing her cock down your throat. She quite literally fucks your face, driving her cock into your throat over and over until she hits her peak. As soon as she starts cumming, she pulls her cock out of your mouth and shoots her cum onto your shit covered face.
After she's finished unloading herself you tackle her into the shit pile, which now covers the whole floor, and position yourself so you can fuck her ass. You push your own shit covered cock into her ass, kissing her as you drive yourself into her over and over again. As you're thrusting into her you scoop up some of the shit and feed it to her, she gladly accepts what you give her, chewing it and swallowing it before opening her mouth for more. As you feed her more and start fucking her harder, Rarity uses her magic to manipulate one of the dildos she brought with her, coat's it in shit and pushes it into your ass. As you fuck her ass, she fucks yours with that dildo, driving the two of you closer to the edge.
As you're getting close Rarity cries out 'Finish in my pussy!' You oblige, and pull out of her ass, only to stuff your cock into her marehood. It takes only a couple of powerful thrusts and you're balls deep unloading your seed right into her womb. You kiss her as she cries out again, her own cock shooting cum onto your bellies and her marehood spasming around your cock. You keep kissing even after both your orgasm's have subsided, enjoying being close to each other while being covered in shit. After a bit you pull out of Rarity and straddle her, putting your soft cock into her mouth, she knows what to expect and quickly starts gulping down your piss as it fills her mouth. Once your bladder is empty, you roll off to the side and she moves so she can put her cock in your mouth. You drink down her piss, much like she did for you, until her own bladder is empty.
The amount of shit and piss falling into the pit slowed down, likely because the event was wrapping up and soon everypony would be leaving. You two had no intention of leaving yet, taking the time to have a proper snack and rehydrate yourselves. You couldn't leave until night anyway, not unless you wanted the whole town to know. As fun as it sounded to streak across town covered in shit with hard cocks waggling, that would land in a lot of hot water, literally and metaphorically. Once night finally came around, Rarity used her magic to clean off the toys and pack up all the things she brought with her. The two of you climbed back out of the hatch and quietly made your way back to her boutique, slipping in the back door and into the downstairs bathroom, in which Rarity had installed a shower stall big enough for the both of you. You two took your time cleaning up, working well into the night, but by the time you finished, both of you were completely clean and fresh smelling. You helped Rarity dry off and then carried her upstairs and to bed, where you joined her for cuddles and a good night's sleep.