Consensual sex in the missionary position for the sole purpose of procreation
Igneous Rock Pie ran the brush in his mouth over the gray hoof-sized, jagged piece of basalt, keeping it clear of dust and dirt. The key to keeping a good soup rock was to keep it pure and prevent it from building up too much filth that the outside world would force onto it. He heard the trap door of the dimly lit cellar open, and he knew it was his wife.
"Yes, Dear?" he asked around the brush handle.
"Igneous, I want a child," Cloudy Quartz, his beloved wife, said.
"Very well," he replied, then returned to brushing as the trap door closed.
That night, Cloudy Quartz went to her bed as usual, heart racing with anxiety. She lay on her back, covered up to her chin by her blanket. Her glasses sat on bedside table and her mane was not tied in the neat bun she normally kept it in. She was nervous about her husband possibly seeing her in such a state, but it was necessary if they were to have a child.
Igneous knocked on her door thrice. She quickly blew out the candle, plunging the room into a murky gloom.
"You may enter," she said as calmly as possible.
The knob twisted and the door swung open, revealing the silhouette of her husband, still wearing his wide brim hat. He closed the door, making him nothing but a dark moving shape, lit only by the faint starlight coming through the window. She could see and hear, even feel his approach to the bed.
"Art thou prepared?" he asked.
"I am," she replied.
She saw moving shadows that seemed to imply he was removing his hat, making her avert her eyes lest the guilt of sin weigh heavily upon her. As he climbed clumsily into the bed, she scooched over to give him room, and he covered himself under the same blanket as her. The two felt vileness creeping over them and up their spines; they had never shared the same bed.
"Very well," Igneous said. "Let us hurry so that we may seek forgiveness all the sooner."
"Let's," she said.
He shuffled around and climbed on top of her, still covered by the blanket. With a chaste kiss on her lips for reassurance, he lowered himself onto her, and she could feel his flaccid stallionhood resting between her legs. Her marehood twitched in reply and she gasped in both worry and confusion as she became aroused for the first time in her life. Viscous fluid began to leak from her nether lips, feeling to her like some kind of demonic salivation. His member quickly became stiff against her exposed belly, only serving to increase the vile flow. As some dripped down onto her sheets, she knew that washing them would not be enough; tomorrow they would have to be cleansed by fire.
"Sweet Mother Celestia, forgive me for what I am about to do," Igneous said. He pulled back and his length dropped, his tip in line with Cloudy Quartz's well-lubricated tunnel to Tartarus. With a grunt, he pressed forward into her, causing her to cry out in pain as her hymen was torn in one quick thrust. The pain was welcome, however, as it reassured her that she was not enjoying the depraved and unnatural act.
Being filled in such an immoral manner brought a blush of shame to her face, and even though she loved and trusted her husband, it still made her wonder, in the back of her mind, how anypony could enjoy this. The depth of her disgrace matched the depth of his member inside her. And Igneous' own guilt weighed heavily on his heart, as he knew Celestia was watching them, judging them from her heavenly throne. Every ounce of pleasure he felt against his will, he buried under double the amount of self-hatred and disgust. He wanted to vomit, but he managed to keep himself composed as he shivered, still on top of his beloved wife whom he was presently violating. Then he decided to end the repulsive act swiftly.
The stallion pumped into her, making wet fleshy sounds both ponies would later wish they could forget. On the third thrust, he grunted as his tip flared, spilling a lifetime's worth of built up seed into her womb, planting it inside her fertile soil. She gasped and panted as her walls involuntarily clamped down on his shaft, milking it for all of its precious load. Then he pulled out of her to both of their relief.
"Let us pray to the all-loving Mother Celestia that you are with child so that we need not repeat this sin ever again," he said. "I shall return to my chambers and pray for forgiveness, and I expect you to do the same."
"Yes, of course, dear husband," she said.
Igneous fumbled his way out of the bed and quickly donned his hat, breathing a sigh of relief as he did. He returned to his room and bent over his own bed in tearful prayer, begging Celestia for forgiveness for what he had done. A solid fifteen minutes passed as he pleaded and wept openly before he felt he had done enough, then he crawled into bed and failed to sleep as he fearfully, repeatedly went over the events in his mind.
Cloudy Quartz had a similar experience. The moment her husband left the room, she emitted a deep and heavy sob, then slid out of bed, the cold wetness on the sheets a chilling reminder of what had just transpired. She got on her knees and begged her dear Mother (who art in Canterlot, blessed be Her name) to forgive her for the evil she had just done. She reasoned with the deity, telling her that it was only to have a child, and reassured her that at no point did she enjoy it.
She relit the candle by her bed and tore off the defiled sheets, bunching them up and throwing them into the farthest corner of her room, then replaced them with clean ones. As her marehood was still leaking, she wiped it with a towel, grimacing as she did, then discarded it in the corner with the filthy sheets.
With the worst of it over, she finally got into her bed with the intention of sleeping, and blew out the candle once more.
Boulder, she thought to herself. If it's a colt, Boulder. If it's a filly... Maud.