It catches him by surprise, how easily the word rolls off his tongue. If he was thinking then it wouldn't have, but as it is he has let his mind still for once and it just happens.
There's an odd flutter in his chest as he hears his voice, but it's true, isn't it? That is what they are now, right? Boyfriends.
The woman smiles at the look of surprise on his face.
"How lovely," she says, and squeezes his arm and he can't quite remember what they walking about except that he referred to John as my boyfriend.
Such a juvenile term. They are not teenagers, left that part of their lives long ago. Partner is better. Significant other. Lover. But John is so much more than a lover - he is a teacher and a guide and a best friend and there are no words to describe what all of that is encapsulated into except husband. But they are not husbands. Perhaps it is a little premature to consider that future though there is no doubt in Sherlock's heart that he would like it. And no other term fits, really, except boyfriend.
Foolishly that ridiculous word makes him grin, and he can feel the blush creeping up his neck. It will do for now. His boyfriend.