There's a familiarity that only comes when you share living quarters with someone; at which point, the normal rules of propriety are suspended. That was his understanding, at least. So what if you walk in on your flatmate while he's in the bath, or vice versa? That happens when sharing a bathroom. The first few times, there's the usual screeching protests, and the red hot flash of embarrassment, but after awhile, seeing John naked or partially naked wasn't anything new. Sherlock didn't care in the least if John saw him naked because why should nudity be something to be ashamed of? When he didn't feel like dressing, he simply didn't, as the denizens of Buckingham Palace could surely attest.
For a doctor, John is a total prude-a prude who buttons all the buttons on his shirt and wears frumpy jumpers for extra measure. He's also under the sad impression that those jumpers are flattering.
Sherlock discovered early in their partnership that there are certain things John won't tolerate and one of them is an open acknowledgement that he owns underpants. This little kernel of knowledge was gained after Mrs. Hudson had done both their laundry and politely delivered it one morning. They had been eating breakfast; John with his tea and a copy of the Times, and Sherlock with a fresh round of nicotine patches and a case of the boreds.
"Good Morning," Mrs. Hudson singsonged. She breezed through their flat and set the laundry basket on the table. John seemed surprised by her sudden appearance. "My! You slept in today, didn't you? Must of been a goodin'. Linus says your silk shirts will be ready on Tuesday."
On top of John's pile sat a pair of red briefs with white lining. John's eyes went comically wide when he saw them, and he then pursed his lips while choosing his words carefully.
"Mrs. Hudson, did you just do my laundry?"
Mrs. Hudson was abashed because why shouldn't she. Sherlock had implied that if she didn't like the mess, then by all means, she could remove it. "Well, yes, it was all over the floor."
John looked at Sherlock as if to say, "How do I explain this?" Sherlock merely shrugged, not knowing what the big deal was. Mrs. Hudson loudly complained she wasn't a housekeeper but clearly enjoyed doting on her tenants.
John smiled tightly while his face turned pink. "Thank you very much, but I'm a grown man and can do my own laundry."
"Oh," Mrs. Hudson made a face and shrugged. "Alright then, I'll just leave it right here."
She meekly left. Sherlock frowned disapprovingly at his new flatmate.
"Was that entirely necessary?"
"Necc-She just barged in and touched my underthings!" John gesticulated wildly and then grabbed his clothes from the basket, being sure to hide the red pants against his chest. "I thought I was pretty nice about it!"
"Yes, a nice lady who breaks into your flat and cleans everything. She's the scourge of London. Do you actually hear yourself?"
"It's not about that! Christ. Okay, fine, I'll try not to be bothered by my complete lack of privacy."
"Oh, for God's sake, she touched your pants. Lessons were learned. The End."
John stares at him flatly.
"John, she's 63 years old with no husband or children. Indulge her," Sherlock takes a bite of his toast and gestures vaguely, "especially when she brings biscuits."
John sighs and says nothing. He goes upstairs to put his laundry away and returns wearing a dressing gown. Sherlock smirks at this. John scowls.
"By the way, what grown man wears pants like those? And why do you even have them if you don't like them? The color is still very bright, so you haven't washed them often. They're quite ridiculous at any rate. Were they a gag gift? No. A girlfriend. Sentiment, so you must remember the affair fondly. She grew tired of your choice of 'underthings.' Must be rather dull. Boxers no doubt. No bright colors."
John exhales deeply through his nose. "This conversation is weird and I'm ending it."
Sherlock shrugged. "I simply obser-"
"No, you didn't. You're trying to get a rise out of me. It's not the same thing. While we're on the subject of boundaries, I should mention that I have them and you constantly violate them. At least Mrs. Hudson doesn't go into my bedroom and hover over me while I sleep! Who does that?"
That had been the day before when John wouldn't wake up and they had things to do. He had gone into John's room and waited. John woke with start when he realized Sherlock was bending over him.
"Aaah!" John was a mix of bewilderment and still not awake. He flailed in his bed and was all eyes when he saw Sherlock. Sherlock grinned widely. "Wha? Sherlock? What the hell?"
"A man was found floating in the Thames, in his swim suit, clearly beaten to death. Let's go!"
"What? What time is it?"
"Dawn."
John had not been impressed and was crabby for the rest of the day.
"There had been a murder. I couldn't wait for you to sleep all day."
"I didn't and I don't, but I do need to sleep and would prefer you not watch me while do. This should be easy for you to understand."
Sherlock sniffed disdainfully. "Sleep is waste of time. I wouldn't do it at all if I could manage it. It's all this time spent doing nothing when I have work to do."
"You're still not getting it. Next time? Knock on the door. Wait until I say you can come in. I don't care if there are hundreds of dead bodies floating in the Thames-knock first, you prat."
Sherlock's mouth worked. "Fine. I'm sorry, John."
"No, you're not."
"I'm really not," Sherlock shook his head, "but I'll do what you ask from now on."
"Good. Thank you."
"Fine, but can I say one more thing?"
"Sure," John sighs.
"We've seen your pants and know that you wear them."
"Oh for God's sake..."
John leaves the table in a huff. Sherlock smiles to himself and butters his toast.