John really doesn't understand why people keep mistaking him and Sherlock for a couple. It just doesn't make sense. As if he would ever date someone as utterly insane as Sherlock Holmes...or a man, for that matter. And Sherlock would never date anyone—married to his detective work, he is.
Moreover, it's not like they look like a couple.
They don't share secret looks or talk to each other with their eyes. They don't order for each other. They don't snatch bites out of each other's food or pay for each other's meals. They don't hold hands (or share any physical contact, really) and they certainly don't flirt.
They just don't do the things that couples do. Yet, everywhere they go it's "You two are so sweet", "How long have you been together?", "You'll be sharing a room, then?" and "Goodness, your boyfriend treats well. You must feel lucky to have him."
Whenever it's said, Sherlock stays silent, distracted and lost in his thoughts. It's John who chokes on his tea and splutters, "We're not a...uh, we're not a couple."
It ridiculous, that's what it is. They aren't a couple and they don't act like a couple and they don't look like a couple and, in John's opinion, there's absolutely no reason for people to think they're a couple.
It's just one of those things that John knows. They're not a couple and therefore, shouldn't be treated as such.
Sherlock, however, knows better.
He catches the secret looks and he knows how to communicate with his eyes. He knows what John likes to eat and he's more than happy to hand over his credit card to pay for it. And Sherlock would never underestimate the depth of touching someone's shoulder, however fleeting it may be.
They do, Sherlock knows, sort of look like a couple. And he'll never speak out when asked because in his experience, if it looks like a duck and quacks like a duck...well, it probably is a duck.