Okay, Spike. I kinda, sorta may have stumbled across this when I was changing the sheets on your bed. I wasn't snooping, I swear. I just saw that you had been eating in bed again and thought it needed cleaning. Needless to say, the content surprised me. Although even then I couldn't really help myself; I figured I might as well try to help out. In retrospect, I'm not sure that it was a good idea. Just read through my comments and then come see me.

The young dragon put a claw to his cheek and gave himself a pinch. A jolt of pain coursed through his body, telling him that he was still awake. Which was odd, because the scene before him was one that had played out many a time in his dreams. An alabaster unicorn mare was sprawled across the bed, her violet mane and tail drifting slowly as she inhaled and exhaled, every breath sending ripples across her coat. Her body was lightly dusted with powdered sapphire, which shimmered in the flickering light of the candles. Her eyes were half-lidded; not from fatigue, but from contentment. The edges of her mouth were just the slightest bit upturned. Her tongue slipped out of her mouth and around her lips.

"Well, what are you waiting for?"

Wow, Spike, this isn't half bad. There are a few descriptions that are a bit cliched as far as romantic literature goes (you've obviously taken inspiration from some rather famous but heavily imitated works), but on the whole you do a good job of setting the scene and describing the object of the narrator's attention. Although it's rather transparent that you're talking about Rarity here, so this could easily come across as rather creepy. In any case the scene itself is pretty good, although it doesn't exactly stand on its own very well. We have no idea how the narrator got in this position. You might want to explain that a bit.

He couldn't answer. There was no answer. He had spent his whole life waiting to see that which now lay before him. There was not a single reason to put it off a moment longer. And yet, he found himself unable to move an inch. It was as if his eyes refused to be torn away from her. Not even a blink. The mare raised a delicate hoof and beckoned him.

"Please, dear. Won't you come?

That's what she said!

...sorry. Back to the story.

He swallowed and took a step forward, finally remembering to take a breath. She responded, gently shifting her body, sending a glimmering cloud of gemstone dust into the air. She faced him, and reclined once more, sliding her forehooves down the curves of her body. A lump formed in the young dragon's throat as her hooves reached her flanks. She pushed against her own hips, gently drawing them apart, revealing the gentle curves of her ovaries.

Aaaaaand we have a problem. I think it's worth noting here that the ovary is an internal sexual organ. As in, it's on the inside of the body. If you ever see a mare with her ovaries visible, you should get her to a doctor right away. There are a ton of other organs that would make a lot more sense in this context. I'd explain it, but I think I need some preparation before I can do the subject justice.

The smile on her face turned lustful.

"Do you like the view?" she asked.

The dragon could only nod.

"Well, don't just look." She raised an eyebrow and scooped up a bit of sapphire dust, and sprinkled it over her uterus. "Care for a taste?"

Under normal circumstances, a mare isn't going to be able to put anything on her uterus. Nor is it a good idea. They could get some serious infections. Again, there are other, better, things you could use here.

He nodded dumbly. Surely it would be the most delicate, decadent, sumptuous flavor that had ever touched his tongue. He knelt, placing his claws gently against the curves of her thighs. She shivered as she felt his breath against her nether regions. He leaned forward. He could hardly breathe as his tongue brushed ever so gently against the lips of her cervix.

No, no, no! Unless she's spreading really wide, there's no way for someone to lick her cervix! And cervixes don't have lips anyway. At least I don't think they do. I'm not entirely sure.

It tasted like pure bliss; every memory of every delicious morsel that he had ever tasted flooded into his mind. This surpassed them all. Sweet, tart, savory; it was every form of every flavor he had ever tasted and enjoyed rolled into one. But the taste meant nothing. What mattered was that it was her taste; that he had the privilege of experiencing her in such a manner.

"Enough of this," she said. "This is far too one-sided. We must belong fully to each other!" She reached out, lifting him up. She put a hoof against his chest and slid it down his boy, feeling the ridges of scales. Lower, lower, she reached a gap in his armor. He began to sweat. She smiled. "We will become one."

He could feel his heartbeat reverberating through his nether regions. Her eyes watched intently as she coaxed his member from its hiding place.

Huh, that's an... interesting fact about dragon anatomy. Although it is common knowledge that reptiles have a genital sheath, so I guess I should've seen it coming. Still, it's just weird to hear you talk about it like that. It's so odd.

She pulled him closer, guiding him into her. It couldn't be real. It was impossible. He couldn't believe it. That they were together the most intimate manner that they could possibly be. That their bodies were within each other. That he could feel her Fallopian tubes against his penis.

Okay, this just baffles me. Pretty much no romance novel ever mentions fallopian tubes, so I have to assume that you learned the term from an anatomy text. But the thing is, every single anatomy book I know has diagrams that illustrate that fallopian tubes are an internal construct. So I'm not sure where on earth you learned the term without knowing what it actually is.

Also, the fact that you refer to the male genitalia in a rather indirect way up to this point makes the last sentence of this section a bit of a mood breaker.

She gasped as he thrust into her, every movement eliciting a moan. They looked into each other's eyes. They needed more. The pony arched her back and began to blow him as they fucked.

Okay, apparently this pony (who totally isn't Rarity or anything) doesn't have a spine, and can also phase through her own body, because otherwise it's physically impossible to give someone a blowjob while having sex with them. Unless another weird quirk of dragon anatomy is having a second penis, in which case I don't want to know. If that is the case, you might want to specify that, because I'm pretty sure not even Princess Celestia would know that. I'm not sure whether she'd want to know, though.

In any case, I'm going to make the somewhat more sensible assumption that dragons only have a single penis, and that you just don't entirely understand what a blowjob is. I'm not about to take the time to explain it, but you really shouldn't try to write about it without understanding exactly how it works.

And another thing, your descriptions are generally quite artistic, so suddenly using a vulgarity like "fuck" out of nowhere really doesn't fit.

He had to return the favor. He leaned forward and began to gently caress her vulva with his tongue.

I guess the dragon doesn't have a spine, either. Okay, at least this is kinda something that could happen. The vulva is something that he could lick. Unfortunately, a certain other part of his body is in the way of that, so in this specific circumstance it's not really possible.

She smiled at him and began to felch as he began to blow her oviduct.

There is so much wrong with this that I don't even know where to start. It's physically impossible to give a female a traditional blowjob, the oviducts, like so many other pieces of anatomy you've mentioned, are internal, and felching is impossible before you... you know what? I'm not even going to start. Needless to say, it makes absolutely no sense in this context. You're having the characters perform a whole litany of physically impossible actions. Anyone who knows even a little bit about sexual anatomy is going to be laughing their heads off at this.

"I'm coming," she said as he had sex with her anus.

Okay, I think I'm going to stop here. After reading this, I think it's high time we actually had a talk about sex. I admit I've been putting it off for far longer than I should have. There's no way it isn't going to be a thoroughly awkward experience for both of us, and I know very little bordering on nothing about how dragon sexuality functions. Although now it seems I really must explain some things to you, because you clearly have some... interesting notions of exactly what sex involves, and I feel somewhat obligated to explain exactly how things work. Although I'm not sure I'm up to it. Perhaps somepony else might be better suited to the job. Maybe Princess Celestia; she more or less raised you after all.

That said, you have a decent ability to write prose; it's quite obviously your first attempt, but I think that if you put the effort in, you could become quite the author. It isn't perfect by any means, but it's better than a lot of the other... stuff I've proofread. But even so, I think you should pick different subject matter to write about until you actually learn a bit more about anatomy. So I recommend that you write about libraries or something that you have firsthand experience with.

A secondary concern is that this seems to be a pretty obvious self-insert story involving yourself in Rarity, which is... to be honest, it's a little creepy. Granted, at least the stuff you attempted to describe is consensual, which is good, but even so I don't think I'd let Rarity see it. Then again, in light of recent interactions with Rarity, she might not think so badly about reading erotic fiction about herself. In any case, you should be careful.

I'm leaving this on your bed, so I know you saw it. I know it'll be weird, but we really do need to talk things over.

Author's Note

I basically just used human anatomy as the baseline for this. I have no desire to look up how horse genitalia function. Sure, it's probably inaccurate, but that's kind of the point.

I figured it would be amusing to have at least one chapter that revolves around a character who utterly fails to know anything about anatomy or what sexual slang means.

Apologies for taking so long to write this. It was a tremendous pain in the ass, and even now I don't think it's quite as good as it could be. But I'd rather move on to something new than spending two weeks rewriting this, so this is what you get.