It's been a few years since Twilight received her castle. Spike has grown up a little bit, and has recently started taking an interest in mares. Not the young puppy love that he originally had for Rarity, though that's not completely gone. Instead, he has been feeling the strong power of pubescent lust.
Twilight has decided that she wants to know more about dragon physiology, and so has asked Spike to participate in a series of experiments. One thing leads to another, and... well, you see where this is going.

Edit: Added a second chapter for the less adventurous among us.
Originally inspired by this story. You'll understand when you read it.
Contains the following: double dick, blowjob, cunnilingus, double penetration, creampie, mild amounts of dirty talk (at least, mild for one of my stories)
Kinkier version contains felching.