(but this is a Boston Thing and it's legally binding)
"Sherlock," Joan said, after shuffling through the mail, "why do I have a very official looking letter congratulating me on my new tax benefits?"
"I'd imagine it's for the same reason I have a very similar letter," Sherlock replied. He didn't look up from the lock he was fiddling with.
"I wouldn't know if you do," Joan said with the weary acceptance of someone faced with something that would have annoyed them five years ago, "because I do not open other people's mail. Unlike certain other people in this house."
"You really ought to get over that little hang up. You don't have a problem with breaking and entering, after all."
"Shh," she said. "My mother still thinks I'm a good little sidetracked former surgeon turned police consultant."
Sherlock did look up at that. "Good god, your mother hasn't snuck into the house in the last hour, has she? I admit I was absorbed in my studies, but surely I would have noticed that."
"Sherlock. Tax benefits."
"We've lived together for seven years, Watson. I believe we have all unwittingly entered into what is colloquially termed a 'Boston marriage.'"
"Boston marriage was slang for longterm lesbian couples, so unless you've been holding back a choice tidbit of information for seven years and would like to tell me your preferred pronouns now…"
"I have held back nothing from you, Watson, and I am insulted that you would think to joke about it."
"Nothing. Really."
"Nothing lately."
"So, Mr. Nothing Lately, your genius intellect and masterful observational skills failed to note the date and laws pertaining to common law marriage in the state and country in which we reside and led you all unwittingly to the end of bachelordom?"
He favored her with his best innocent face. In Joan's professional opinion, it needed work. It was something around the corners of his mouth. "I believe we are partners, Ms Watson. Does that not make us equally responsible for these sorts of things?"
"One of us has an encyclopedic memory of New York law and one of us has to refer to her sticky-noted reference book," she retorted. "Fess up."
"Think of how much easier it will be when the inevitable complications of investigating hardened criminals ensue!" he protested.
Joan 'hmm'ed and looked at the letter again. "And there are some nice tax breaks involved."
"I promise not to tell Detective Bell that we are legally married," Sherlock said. "I think that would put the poor man off."
Joan glared at him with all the righteous fury she could muster, which, admittedly, wasn't much. "You are not the man I met seven years ago."
"You love it."