You know what’s just amazing?
John loves Mary, so Sherlock loves Mary. He’s so fiercely protective of her and their relationship because to be protective of her is to be protective of John.
And now they’re all like, “You’re gonna have a baby!” <3<3<3
But then there’s this theory going around that Mary’s baby is not John’s. That she’s having an affair or, like in ACD canon, had a previous relationship where something happened, or…something. And you know, things support that, like her initial reaction of fear (not when she passes into more normal ‘I’m gonna have a baby!’ panic, but the VERY FIRST expression on her face when Sherlock tells them). And then, the whole episode there was this ‘elephant in the room’ gag.
Joolabee made a guess that not only is John not Mary’s baby-daddy, but that Sherlock knows it.
So. What if Sherlock’s little downcast look and then him leaving there at the end isn’t because he’s pining, but because, in the wake of his vow, he’s decided to go deal with this little problem before it bothers anybody?