Regret and Other Mistakes: John challenges Sherlock when he really should have known better. (Rating: M; Slash.)

They'd had rows before, of course; it was nearly impossible for two so very different men to live together without a few terse words every now and again. This was different though, this was the kind of anger that made you want to screw your whole face up and scream, and it was coming from Sherlock who was usually so above the banalities of emotion.

"Did it ever occur to you that I would want you here this evening?" the dark-haired man seethed, pacing the length of the front room like a caged animal.

John just stood in the hallway, feeling wearier than usual. The minute he'd come home from his date with Sarah, Sherlock had attacked him. He'd known that the younger man didn't agree with John's decision to date, hated the time the doctor spent away with all the petulance of a spoiled child, but some perverse part of him had enjoyed watching the younger, aloof man squirm. Usually, he would relish these all too human outbursts from his flatmate, only tonight the other man was visibly angry and John was already drained from fighting with Sarah. Tonight, his temper was too close to the surface.

He snapped at the accusation, an instant sneer curling his lips, "Yeah, well, you can't get everything you want, Sherlock."

Sherlock stilled, his face going blank, and it was at that moment John realized this was a challenge he would later come to regret issuing.

For the first week after that, John walked around as though waiting for an explosion to go off, but Sherlock acted as if the incident had never happened. Parts of John were grateful for this reprieve as it gave him time to patch things up with Sarah, but other parts of him were deeply suspicious. It wasn't like Sherlock to simply give up on something- what was he planning?

"You're being paranoid," Sarah told him plainly.

"No. This isn't normal for him; he's up to something," John shook his head. "You don't know what it's like living with him."

"Sometimes I feel like I do," Sarah replied sourly, "I certainly hear enough about him."

He suppressed a sigh. "Don't start that again-"

"John," she interrupted him warningly, "it's not healthy, the way you're going on about this. Your whole world revolves around Sherlock, and I worry that you're losing sight of everything else. It's all well and good to play detective, but not at the expense of a real life."

He knew this was a conversation they'd had more than once and that no one could win; he knew that he should just walk away, but he was tired and edgy and his mouth shot off without stopping for permission. "Real life? You mean sitting in a cramped office, listening to eighty year-olds tell me I've under diagnosed their cold? Or how about getting stiffed for change at the grocer's by a bloody machine? Sitting in my flat, reminiscing about things that can never be changed? That's not the life I want or need." He ran a had over his eyes. "I don't play at detective; I take the opportunity to save lives and bring murderers to justice. That's real, Sarah."

She gave him a dead-eyed stare. "Then what are you here for, John?"

The question echoed around his thoughts, leaving him with the sinking feeling that he had an answer but didn't want to acknowledge it.

The next day found John just a hairsbreadth edgier, and seriously questioning his relationship with Sarah. It also found him in a pub which, by coincidence, happened to be one of the favorite haunts of Sergeant Sally Donavan.

He liked Sally. She was gruff and a little mean from her time on the job, but she had a good sense of humor and cared more than she let on. And she liked him, in a 'business friends' sort of way.

"John?" she asked, approaching him with a cocktail in hand. "What're you doing here?"

He was stewing fairly deep in anger at that point, so it took a decent amount of effort to keep from snarling. Instead, he snapped, "It's a pub, Sally, what do you think I'm doing?"

She raised a dark brow, a comeback springing to her lips instantly, "Going on your tone, I'd say menstruating."

And just like that, John's anger fled him. "Sorry, I don't mean to be rude," he apologized, gazing morosely at his latest pint.

"I don't take offense," she shook her head, sitting down next to him. "Living with someone like Sherlock Holmes, you're bound to be tense more days than not."

He didn't want to talk about it, didn't even want to think about it anymore, but he was pretty deep in his cups and he'd apparently lost control of his mouth a few drinks ago. "I'm at my limit, and I've got no idea how to fix this."

"What's he done now?" Sally asked resignedly.

"Nothing," he replied in exasperation. "That's the trouble: he isn't doing anything."

Her brow furrowed. "I don't follow."

"We had an argument a week ago, and he's acting as though he's forgotten all about it," John explained, his frustration clear.

"He does that," she reminded him gently. "You said so in your blog- 'deletes' everything he doesn't find useful to his detecting. Besides, it was a week ago."

"Doesn't matter," he shook his head. "If there's one thing I've learned about Sherlock it's that he can't help but push at people's boundaries." He took a pull from his beer, continuing in a low tone, "He wouldn't have forgotten this; it's too good of a chance for him to make me uncomfortable."

"I warned you off him, didn't I?" Sally reminded him. And, for a moment, it seemed as though she was going to leave it at that, but some foreign sense of compassion obviously seized her. With a sigh, she asked, "So what's this really all about then?"

John considered his words carefully. It was difficult to explain what was wrong without making both him and Sherlock sound completely dysfunctional. "He's never really had a friend before, not one that he actually wants to spend time with," he replied after a few moments. "I'm a novelty to him, a commodity he no longer wants to do without. He makes demands on my time that no sane person would even suggest. Normally, I wouldn't complain- my life's a bit on the dull side as it is…"

She looked as though she knew where this was heading. "But?"

"Sarah," John sighed. "He sees her as competition. And no matter what I do, I can't make him understand that me being involved with her makes it necessary to spend time away from him."

"Maybe," Sally frowned, fiddling with her cocktail glass, "he doesn't understand romantic attachment?"

"Trouble is, he does," the doctor shook his head. "Better than he ever lets on. And that adds a whole new shade to this problem."

"You think he's attracted to you?" she asked him quietly, but there was no real surprise in her tone.

"Not sure," he shrugged. "But it would make sense, if you think about- me being Sherlock's first real emotional attachment in years."

Sally hummed an agreement, then asked, "What does Sarah think about all this?"

"At the moment?" John shrugged, feeling a headache coming on. "Who knows?"

"Oh?" she cooed, a slight smile tugging at her lips. "Having a bit of a domestic?"

He frowned at her, not finding it amusing in the least. His social life was in complete shambles, and it was all Sally could do not to laugh at him. "Thanks to Sherlock," he replied grimly.

"He talked to her?" she guessed.

"No," John shook his head, "but apparently she feels that I do little more than talk about him, that I just wait around for him, willing to drop whatever I'm doing if he calls."

"Well, it's true," Sally chuckled.

"I know," he sighed. "That's the problem. I want to be mad at Sarah for saying so, but I can't really deny it, now can I?" It was a hell of a problem, too, to be torn between two so very essential people. Sherlock gave him all the adventure and excitement he'd been missing, and Sarah gave him the affection and physical contact he craved; he couldn't do without one or the other, yet they both seemed determined that he should have to choose between them.

"Seems to me," Donavan interrupted his inner ramblings, "that you need to worry less about what he's thinking and more about what you are. Would it bother you if Sherlock took Sarah's place? Would you miss her if he did or just find it easier to have a sex life?"

It was a disturbingly simple solution to his problem. He'd considered it before, of course, but it was hard to read his flatmate's possessive behavior. He was never sure if making an overture would be understood or even wanted. For all he knew, Sherlock was completely asexual and capable of nothing more than a platonic friendship.

Then again- and maybe it was just the beer talking- John would never know unless he tried.

"I'm not recommending you take things up with the freak," Sally continued, "we all know there's a world of problems that would come about from it. But if you're always going to be hanging on his every move anyway, what difference does it make? Did you ever stop to think that, maybe, you're on the wrong side of the relationship?"

That was just it, wasn't it? In most ways, their relationship already resembled dating. Nearly everyone who met them thought they were involved- in fact, Mrs. Hudson had bets going with both Angelo and the landlady next door as to when the two of them would admit they were going out. Really, when John thought about it, the only thing he was getting from Sarah that he wasn't getting from Sherlock was sex.

That thought made him uncomfortable. Not the thought of sex with Sherlock- although he'd never actively thought about having sex with another man before. No, the thought of Sarah, because he finally couldn't escape it any more. He'd been using her for a taste of something normal and to fill in the gaps that Sherlock had left behind. Sarah was beautiful and loving woman, and he enjoyed their time together, but he had to admit that it probably wouldn't bother him at all if Sherlock took her place.

It was a revelation that was both late in coming and not really a revelation at all. Still, it made John feel a little better to have things sorted out. Now his only problem was approaching Sherlock about it.

Several hours later, John walked through the front door of 221B, his nerves stretching tight as he climbed the stairs. In the end, he'd decided it would simply be easier to kiss Sherlock and wait for a reaction, that way he couldn't mince words and wouldn't have to worry that his flatmate would misunderstand him. Didn't put him at ease though; fear of rejection still made him feel like he was heading off to battle.

The front room looked as it always did: an absolute disaster with Sherlock sitting at it's epicenter. And Sherlock, well, he looked as he always did, too: neat and dark and just the tiniest bit expectant. It occurred to John that the man was waiting for him, that somehow his flatmate knew what he'd planned, but then that was the curse of living with someone so clever. Those pale, all-seeing eyes studied him with the curiosity of someone watching a caged animal- appreciative, but wondering what would happen when the animal broke loose.

"You're home late," Sherlock observed lowly, his voice deep and smooth. That dark baritone had always sent something vibrating in John, he'd just hesitated to put a name to it. Now he knew, now he couldn't ignore.

He didn't respond, didn't think he had the strength to form any words. Instead, he approached Sherlock, only hesitating slightly when the younger man stood. But the hesitation soon faded away, and John slipped into that curious calm that came over him when he was firing a gun: his breathing slowed, his limbs went steady, and his mind emptied of everything but his target. There was no time for panic, no reason for panic; he had one objective, one goal, and he would complete it with all the precision of the soldier and doctor he'd once been.

John kissed Sherlock.

It was a little awkward at first given their height difference and the fact that he'd never thought he would kiss another man, but Sherlock's lips were soft and pleasant, and the embrace soon evened out.

They pulled apart, Sherlock's eyes narrowing as a dark brow rose. His hands, thin but deceptively strong, moved to John's shoulders, griping him loosely. "Declaration or experiment?"

"Excuse me?" John blinked, his calm fleeing him. Objective complete, but now he had to make sense of it- was that kiss worth the life they had already built together? His mind was understandably elsewhere, but when the other man's words finally sank in, he had to fight down a blush. "Little bit of both."

A smile tugged at the younger man's lips. "Conclusions?"

"I don't really have any yet," John replied, flustered.

Sherlock's hands performed an intricate dance, one slipping to his flatmate's waist as the other curled around his jaw. Strong fingers tipped John's chin up as the taller man murmured, "Let me help you decide."

This kiss was different. It was intense and consuming, passionate and just a little bit wild. Sherlock's lips worked powerfully over his own, guiding them away from the chaste embrace of earlier into something new and addictive. Their bodies strained together, closing the meager distance between them, a fire lighting between the two men.

John let his thoughts slip away, let his worry die. This kiss, this single embrace put to rest every question he'd had. Yes, he fancied Sherlock. Yes, Sherlock fancied him. Yes, sex was likely, and they would probably both enjoy the hell out of it.

The detective, sensing John's brief inattention, redoubled his efforts. His lips pulled possessively at the other man's mouth, coaxing deviously until he'd gained entrance. And that was really all he needed, because once that gate was open they were both fighting to one up each other and the doctor couldn't spare attention to anything else for fear of losing.

They broke apart with no clear victor, each trying to catch their breath and seize up their opponent. This was a critical moment in John's mind- whoever launched an attack first had the upper hand, and he knew it needed to be him. If things with Sherlock progressed, if they really entered into a romantic relationship, what few barriers the detective had allowed him would be ruthlessly torn down. Without those barriers Sherlock would control everything, unless John established some amount of dominance now. He didn't want to be master of the relationship, he just wanted to make it clear that he had to be the detective's equal in this matter.

The soldier in John reared his head with one intent: victory. Bypassing thought, just in case Sherlock was being his usual observant self, John hooked his leg behind the other man's knees and sent them both tumbling to the ground. It was the perfect position really, because it eliminated the height problem and put John squarely on top of his flatmate.

Sherlock didn't even have the decency to look surprised. A wicked smile curled his flushed lips, "Bit eager, are we?" He stretched, a calculated and purely sadistic move that ground their hips together.

"Fuck," John hissed, nearly doubling over. And that one moment of inattention, of promised pleasure, was what the other man had been waiting for. The world spun in a blur and John suddenly found himself on his back, Sherlock's taller frame pinning him to the ground.

"Are you sure about this?" Sherlock asked, his tone gentle even as he continued to grind their hips together. "You won't regret it tomorrow?"

"I'm regretting it right now," John growled, impatience fueling his building desire, "by tomorrow I'll be reconciled."

Sherlock sat back a little, just a little, but it was enough to get a hand between them. His clever fingers began to trace the shape of John's eager member through his trousers. "I'd hate to get between you and Sarah," he murmured, not even bothering to disguise his glee.

John didn't answer for a long moment; too long, apparently, because the detective's fingers stopped. His hips bucked involuntarily, but he found himself with no where to go. "You're a bastard," he snapped.

"Indeed," Sherlock smiled, giving one brief stroke before pausing again. "Still…?"

"There is no still, Sherlock," John replied, his brain slowly shutting off as his world narrowed to the simple want of pleasure. "I think things have gone about as far as they can."

His fingers began moving again, undoing the doctor's fly to delve for the organ hidden underneath. "Not quite," he smiled, his thumb flirting with the head of John's cock.

John's world exploded, and just when he was starting to make sense of things, he felt a mouth close around the crest of his desire. He didn't last long. How could he? Sherlock watched him, those pale eyes trained to his face as that dark head bobbed along his length. How was he meant to hang on while those eyes spoke of wicked promises; while Sherlock exploited everything he knew about male pleasure; while the devil himself played his body like a bloody violin?

He came violently, a nearly inaudible scream rushing from his lips. It was the single most painful and pleasurable moment of his life; it eclipsed every fumble and every thrust of every relationship he'd ever been in. It was terrifying and wonderful, and the man who'd done it to him was biting at his hip and smirking. So, of course, John felt obliged to reciprocate.

Getting Sherlock under him was easy- mostly because the detective let him. The hard part was in figuring out what to do. He decided to start at the neck, flicking out licks and kisses, sucking here and biting there until the man underneath him bucked. Quick hands opened Sherlock's shirt, revealing the man's pale chest as John quested south. He concentrated on one spot at a time, always staying long enough to frustrate the other man before moving on. John took his time, exploring and torturing, his every move both a thanks and revenge for the magic the younger man had worked earlier. By the time his lips finally wrapped around Sherlock, the detective was already growling his name like a mantra.

A silence followed their frantic coupling, hushed but for the echoes of moans and screams. John was just starting to get the slightest bit uneasy when Sherlock wrapped an arm around him.

"Sarah?" he asked, his deep voice vibrating through John, who was still more or less on top of him.

John considered for a moment. "She's a wonderful woman, but she wanted more from me than I was able to give her. She wanted all of me, even the parts that hang on you and, well…"

"They're mine." Sherlock drew him closer.

"She didn't understand that," the doctor shrugged, laying his head down on his new lover's chest. "Hell, I didn't understand it until Sally pointed it out."

"Donavan?" For once, Sherlock sounded surprised.

"Yeah," John couldn't help but chuckle as they both took a moment to marvel at the thought of the prickly sergeant giving relationship advice.

"You'll have to break it off with Sarah, you know," the detective told him firmly.

"I already did," John sighed. He'd wanted to enter into things with a clear conscience, and the revelation that he'd been using Sarah as a surrogate hadn't felt right. "Granted, I might have to do it again, because she knew I'd just come from the pub."

"She also knew it was about me, so what difference does it make if you were sober or drunk?" There was a careless drawl in Sherlock's tone, but his other arm came up to pin the doctor to him. He was clearly not letting go, especially not for Sarah.

Another silence reigned, comfortable as they explored the new physicality of their relationship. Of course, silences in 221B were always short lived. It was John who interrupted it this time. "You did nothing all week; you never do nothing," he said accusingly, lifting his head to stare at the younger man. "You were playing mind games with me, weren't you?"

"Only a little," Sherlock replied, ducking his head to taste the doctor's neck. "I knew you would do all the work yourself. It was just a matter of time before you acted."

John fought down a moan. "I thought you considered psychology a soft science."

"I'm not above using it when it gets me what I want," Sherlock breathed out in triumph, nipping at his throat.

A/N: This all started from a line I pulled out of one of the other stories, and it turned into a complete monster. I'm not entirely thrilled, as it's been a long time since I've written romance.

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Disclaimer: I do not own BBC's Sherlock, and I'm not making any money off these stories.