A/N: I have no idea where this came from, but there it is. Not a genderswap story in spite of the opening question. Rated T for sex-talk.
"Sherlock, what's the first thing you would do if you woke up one day and found you'd been turned into a woman?"
Without looking up from his microscope Sherlock replied, "Have sex with Molly."
The sound of the newspaper rustling to the floor and a sort of strangled gasp from John's throat told Sherlock his response had met with the expected reaction. He grinned to himself as he adjusted the focus on his mold sample. "Not the answer you were expecting, then?"
"Uh, no," John replied, sounding more than a bit flustered. "I was actually expecting to be told not to be an idiot."
"Yes, well, you happened to ask a question I've thought about once in a while," Sherlock replied, his grin turning to a scowl as the sample did not exhibit the attributes he'd anticipated. Drat, another experimental dead-end.
He looked up, reaching for another slide, and saw John standing in the kitchen door, staring at him. "What?"
John shook his head. "You've thought about it. Getting turned into a woman."
"And having sex with Molly," Sherlock agreed with a smirk. "As a woman, I mean; I've already had sex with Molly on several occasions as a man, and I'm sure she'd be willing to let me…"
"Wait, hold on, back up!" John exclaimed, a scowl now twisting his features. "You've had sex with Molly? When?"
Sherlock shrugged as he crossed the kitchen and pulled open a drawer at random, having no idea where he'd put the box of slides he'd filched from St. Bart's two weeks ago. "Right after I jumped. Shortly after she ended her engagement and I stopped taking drugs. In the hospital after I was shot, right before they discharged me. Quite a lot since Moriarty was dispa—aha!" he interrupted himself, exclaiming in triumph as he found the box and carefully pulled out a new set of slides. He flashed a grin at John, who was staring at him, utterly flummoxed. "What, didn't you know? Mary does, she guessed it right away," he added, unable to resist tweaking his friend.
"So you and Molly Hooper are…together," John finally managed.
Sherlock nodded. "Yep," he replied, popping the 'p' at the end of the word. "Would have been together as soon as I returned from the dead if she hadn't been too busy convincing herself that shagging meat dagger would make her happy. Luckily she figured it out in time."
John shook his head. "You and Molly Hooper. Together. Having sex." He shuddered. "Nope, can't picture it…WON'T picture it," he added fiercely, as if arguing with his imagination…and failing. Sherlock didn't bother hiding his smirk at his friend's discomfiture; he'd been waiting a long time for this very moment, and was quite pleased at John for so randomly introducing the topic of sex. Well, he hadn't intended to, of course, but that made it even better.
The sound of Sherlock's front door opening caught both men's attention; John turned, and Sherlock hurried over to relieve Molly Hooper, the very object of their discussion, of the loaded Tescos' bags she was carrying. "Thanks," she said, grinning at him. When she noticed John, she gave him a smile and a small wave as she divested herself of her jacket and handbag. "Hello, John! Will you be staying for dinner, or are Mary and Isabelle expecting you home?"
"Uh, no, I can stay," John stammered out, once again looking and sounding decidedly on the back foot. He and Mary had gone away with their two-month-old daughter for two weeks – only two weeks! – and apparently the world had turned topsy-turvy. "I'll just let Mary know."
Sherlock, however, was shaking his head and grabbing for John's coat, having deposited the groceries haphazardly onto the opposite end of the table from his precious microscope. "No, John, you can't stay, remember? You told me Mary had something special planned for dinner if we finished up the case early, which we have. So sorry you can't stay, do give her and my goddaughter my love and of course Molly's as well, yes? Ta!"
With that stream of words, John found himself out on the landing, coat half on, his face the picture of bewilderment as Sherlock shut the door. And locked it; he heard the lock turn, and his confusion turned to embarrassment as he heard Molly's giggle and the thud of two bodies pressed up against the door, followed immediately by a series of moans.
"Right, then," John said. "Off I go. Home. To my wife. Who maybe can explain this to me."
He shook his head and clattered down the stairs, bidding a distracted good-bye to Mrs. Hudson, who was just coming in the front door.
It took the entire cab ride home for him to rid himself of the mental image of a female Sherlock snogging the daylights out of a confused but delighted Molly Hooper.
Some questions were just better left unasked.