AUTHOR'S NOTE: I realize that in the books (and possibly in the TV show, I don't think it's been discussed) Jim was older than Sherlock. Head canon says otherwise. Just roll with it.
James Moriarty didn't look anything like a criminal mastermind when he slept, Sherlock mused. He was... softer, somehow, the lines worn around his eyes smoothing out, an almost peaceful expression on his face. He nuzzled into Sherlock and tugged away the duvet. It was sweet, really- the man who so hated domesticity being just so domestic. Sherlock almost hated having to break up the moment, but nevertheless, he reached out and scratched Jim's stubble.
Jim mewled and opened his brown eyes slowly. They focused on Sherlock and he smiled. "Good morning, gorgeous."
Sherlock stretched and gave him a look. "You know how I feel about nicknames, Jim."
Jim rolled his eyes. "Yes, but I've chosen to ignore that... dear." He flashed a smug, provocative grin. He began to get up, moving as if every muscle hurt him.
Sherlock looked concerned. "Are you all right?"
Jim tsked. "I'm fine. Just old and you've been running me everywhere." He walked to the kitchen. "Fancy a coffee?"
Sherlock laughed sharply. "You're younger than me, Jim."
He rolled over in the bed. "No, what I really want is you to get back in here and we can continue what we started last night."
Jim grinned. "That does sound... tempting." He tilted his head. "But God, have we become domestic."
Sherlock shrugged and motioned again for Jim to join him. "That might not be such a bad thing."
And indeed, as the two men met in a flurry of limbs and lips, Jim had to agree- he could really get used to domesticity.