He's helping Mina decorate the Christmas tree, all the while lending an indulgent ear to her cheerful babbling; her talking skills are improving daily now, and he's proud of her as if she were his own daughter – he's been living under the same roof with her parents since before she was born, and he supposes that must count as something.
John enters the living room just when they're about to finish; he watches the two of them with a fond smile, then ruffles his baby girl's curls as she trots away in search of her mother.
"She's got everyone wrapped around her little finger, and she's not even two," John says in a mock suffering tone. "I don't even want to know how we're going to survive her teenage years."
Sherlock turns a pensive gaze on his best friend and platonic life partner. "Well, I think you and Mary are right; giving her a baby brother or sister is a sensible idea, even more so while she's still quite young."
"How did you...?" John starts, only to trail off mid-sentence. "Actually, never mind. I should've known you'd deduce it before we got the chance to ask you."
"Ask me? You don't have to 'ask' me, John; I thought that was clear enough."
"Of course we have to. That's how relationships work, remember?"
"Our current living arrangement doesn't change the fact that only two of us are married together, and having sexual relations. You hardly need to consult me on family planning or any related subject."
John rolls his eyes as if he can't believe he has to go through this all over again. "Sherlock, you're our partner; we've mutually agreed on that, yes? Which means you matter to us just as much as Mary and I matter to one another. If we're going to have another baby, we're going to have it all three of us together."
Realisation hits Sherlock full force, and he mentally chastises himself for being so slow. "You want me to father your child."
"Glad we're on the same page at last. Yes, that's our first option; provided that you're up to it, of course. Either way, we're not going to try for another child if that's not what you want."
Sherlock wills himself to breathe amidst the confused feelings that are bubbling up inside him. "I don't do that, John. You know I don't."
"And you know there are other ways, right? If you want, that is. It's your choice, and I promise we'll still love you regardless of your decision."
He stares at the Christmas tree, all shiny baubles and red garland and tinsel, and lets out a shaky breath. The idea of a child of his own is both terrifying and enthralling, and he finds it hard to believe that John and Mary love him enough to give him as much; not when he's the non-romantic, nonsexual party in a partnership that is unconventional to say the least.
"Are you sure you'd be all right with that?" he enquires at length. "Wouldn't you rather be the child's biological father?"
"Quite sure," John smiles; and for all that he's never been particularly fond of affectionate gestures, Sherlock briefly experiences the urge to hug his best friend.
"We don't tell Mycroft though, do we?" he mutters by way of a joke, certain that John will take it for the 'yes' that it actually is.
In the end it's John who throws his arms around him, his merry laughter the most beautiful sound Sherlock has heard in his entire life. That, and Mary's delighted chuckle as she enters the room with Mina in her arms.