Mark was out of his league, fooling around with an immortal pony princess, and he knew it. To make life easier for himself, he accepted that as a undeniable fact. She would get bored and he would move on. He just didn't know that the great vain mare was called the Subtle Sun for a reason. And that her enemies called her that as an insult.
Princess Celestia didn't have a subtle bone in her body.
Currently rated T but I might bump the rating as the story progresses.
Now a one-shot.