Sherlock doesn't like John's army mates. They're boisterous and the vulgarity is repugnant; and he's revolted by their tendency to initiate spontaneous arm wrestling competitions, random headbutts, and generally behave like a bunch of barbarian savages.
The fact that their IQ level is low to average (at best) doesn't prevent them from having conversations that are absolutely over his head, due to their use of slang, military jargon, and countless references to their joint past. To be honest, most of the people he meets share a considerable number of conventions that he never understood and it's never bothered him; but in this case it's different, because it has to do with John.
They express possessiveness towards his John ("Johnny boy" is the commonest nickname, which makes Sherlock grit his teeth. Only he is allowed to pet-name John. He never does it, but all the same). And apparently John cares a lot about them, too. Sherlock figured that out after he deduced out loud an embarrassing detail about the most annoying one: John turned red, seething and looking even more humiliated than the man himself. He apologised immediately and made Sherlock apologise as well.
When they got home Sherlock was still sulking and embittered, but John was calmer, though serious and restrained. "Those people are not just my friends, Sherlock," he'd tried to explain. "It's… much deeper. The army is like another planet; you're forced to experience impossible, ground-shaking things. And you get through it together, leaning on each other and sticking together to survive. The closeness that's created is outside the normal world." Unsurprisingly, that only made Sherlock feel worse.
The fact that John looks untypically loose and light at these evenings vexes Sherlock, too. Usually John's reactions to mentions of his past in the army are troubled: a shadow crossing his face, a crease between his brows, an unconscious touch of his scar. But when he's sitting with the men who are a living reminder of his past, he doesn't seem bothered at all. Moreover, the civilised, well-mannered doctor shows a rough, blunt side that fits easily into the plebeian atmosphere. The reserved, peaceable John, who prefers to hold back and not encourage a row even when daily coping with an impossible spouse and flatmate, looks completely serene in the company of a bunch of militaristic imbeciles.
When they talk about life in a military camp and reminisce about snakes in the loo and scorpions in boots and capricious commanders, John jokes, "I didn't know then it was only basic training for living with Sherlock," and Sherlock flinches a bit as the others burst into thunderous laughter. He's used to John's (justified) remarks about the difficulty of sharing a flat with him, but somehow in this context it hits a point of insecurity. Later that night he gets rid of a particularly horrid experiment that he's kept under the bed without John knowing.
"I told you, Sherlock, you don't have to come," says John for the thousandth time, as they prepare to leave for the pub and Sherlock won't stop complaining.
Sherlock doesn't answer. He doesn't want to say that he's apprehensive about John going there without him. He knows the army did a lot to strengthen John's bisexuality, and along with the intimacy caused by serving together, well, he just doesn't want to risk it. He decided always to attend to serve as a constant reminder that John's taken; even if by someone who's arrogant and condescending and spends the entire evening looking aloof silent (except for frequently correcting grammatical mistakes).
The first time they arrived at the pub together, the others were already sitting at the table (they were late, of course they were, thanks to Sherlock). They were welcomed with loud greetings: "Hey, Johnny!", "Watson!" and one dramatic "Oh Captain my Captain''. John introduced Sherlock, and after a short moment of silence two of them whistled their appreciation. "I'll be damned," the third said. "Now I see why it took so long to set a date for the meeting. If that's what I had at home, I wouldn't have been keen on going out, either." Sherlock tensed, tightened his hold on John's hand and shot them a glare to clarify that he was taken. By John.
A while later he realised it wasn't necessary: despite their initial enthusiasm, there's no doubt that he isn't much to the liking of John's friends - that he's too sophisticated, too elegant and spoiled, and too… gay. It's different for John - seems that John's being bi makes him even more attractive, gives him a halo of experience and success with both genders. They reacted to Sherlock In the same way they might drool over a model in underwear advertisements but eventually prefer someone who'd join them in drinking beer and be accommodating in bed. But above all, it's clear that they'll never do something like stealing a spouse from an old friend - loyalty, at least, is a quality in which they excel.
"I hope he's less snobbish with you than with us, mate?" one of them had asked John with honest care, as if Sherlock wasn't right there. John had offered a short, discrete smile, and didn't answer. He didn't have to. They'd cheered and laughed and slammed the table ("Three continents, and now Antarctica!"), until John hushed them.
But even though Sherlock has difficulty grasping social situations, he's not entirely obtuse or oblivious. His scientific side won't stop examining data and deducing, and he never misses or ignores any of it. Even while he's spending time with his comrades, John doesn't neglect him. He shows affection with their usual public gestures, ruffling Sherlock's curls or catching his eyes for a private moment. He breaks off to explain inside jokes and tries to involve him in the conversation. He talks about their cases, elaborates on Sherlock's genius, compliments him sincerely and uninhibitedly, openly proud of him.
And Sherlock knows that 221b Baker Street is another planet as well, theirs alone. They too face John's PTSD and Sherlock's dark moods together, not to mention the everyday touch of crime. Their relationship includes the physical and emotional intensity of soldiers in battle, as well as romance. And no matter how much John nags about body parts in the kitchen or the violin being played after midnight, Sherlock knows he'll never really try to change him.
He compares his feelings for the so-called "intruder mob" to his feelings towards other social gatherings that John's dragged him to - with Lestrade, Stamford or even colleges from the surgery. And he works out that he just never saw John around people where his relationship was so personal. Although Sherlock's possessive instincts and his demanding personality make him want to put his arms around John and declare that John's his, most of the time he's sensible enough to acknowledge it's good that John has close friends he's happy to be with. Sherlock just can't make himself regard them with anything but a slight recoil and distaste.
He still joins John when he's meeting them. Not every time, but enough to satisfy his minor unease.
One night, John goes to get another round, and two of the company have decided, for their own ape-like reasons, to get down on the pub's less than salubrious floor for a push-ups contest, so Sherlock finds himself alone with the third. He has no idea what his name is - on the first night John introduced them, their full names and nicknames, and Sherlock deleted the information a second later. Of course they have nothing to say to each other, and eventually they're just looking over at John, waiting for him to come back.
"You know, Johnny saved my life," says the man suddenly, sounding a bit drunk at this stage of the night. "Not, like, physically, cos I always managed to get out without a scratch, somehow. But people went completely nuts so far from home, even before the fighting started, and after that it got worse. But not Johnny, he was always so stable, so self-controlled, so… fucking sane. You could always go to his tent and talk, and he'd listen, never run out of patience. And he was always willing to help, whatever he could do, he never said no. Hell of a guy, really." He took another swallow from his drink, eyes still fixed on John's back. "And he went to uni, was a bloody doctor, but he was never stuck up." Sherlock shoots him a sidelong glance, but the man's not giving him a thought, let alone bothering to criticise him. "War is shit, mate," He looks at Sherlock, sobering for a minute, treating him as an equal, then tilts his head back at John. "But that bloke over there? I'd go back to war for him. I'd kill for him."
Sherlock keeps quiet and they both watch John who's smiling in the distance. And at least for one moment Sherlock knows that he and this illiterate, awfully ordinary man are the same.