Disclaimer: I don't own, you don't sue. Tumblr suggested the idea of gay John being flatmate to Fem!Sherlock. If it started from you, drop me a line and I'll happily credit you.
Sheryl
It's an experiment. It's an experiment. It's just an experiment. No harm done if it fails. John might huff about being used as unwitting guinea pig – again – but nothing will change between them. Now, if only Sheryl could convince herself of it, perhaps her stomach would settle. She's not giving herself away, is she? John is a bit of an idiot, and he believes her far too much. He'll believe her again. She's just trying to see if she can pass for male. If someone who knows her so well is fooled, she can go undercover as a man anytime. If only things were so simple.
They should be that simple, because she's married to her work. She has no time for distractions, the empty complexity of courtship rituals, or messy, confounding feelings. She can flirt – she will do so if the investigation requires it – but she won't let silly things like relationships disrupt her life. The gay flatmate looked like a blessing. John's protective streak, his medical prowess, his urge to just care – even for an impossible individual like her… She gets all that without having to pay for it with her body. John gets annoyed by everyone's assumption that they're together (others expect her to be doing at least something right to have him stick around), perhaps because of the mentality underneath it, perhaps because it diminishes his chances to pull cute boys once they've seen her with him. He states his sexual orientation loudly, exasperated, each time someone implies something about them.
John is a blessing, until he becomes a problem because he's too good, damnit. He's brave and funny and gentle and aesthetically pleasing and he accepts Sheryl and he tells her off when needed but never ever saying, "you disgusting freak!" or even implying it. Sheryl doesn't deserve him – oh no, she doesn't even deserve to be in the same room with John Watson – but she might be falling a bit - a lot – fine, she's fallen from heaven to the deepest pit of hell for her flatmate and isn't that the perfect recipe for disaster?
She should just give up, but tonight John Watson is on the pull and if she can find him drunk enough maybe he'll pull her. If she can trick his subconscious, she might have a quick, dirty something with him to masturbate to for years to come. It's selfish. It's manipulative. Just like her.
She binds her chest, changes her usual style a bit and thanks a God she doesn't believe in for her rather androgynous looks to begin with. Years of smoking ensure that her voice is far from shrill. And yes, she can pass for male because once inside the gay bar John favours she has to fend off suitors with a stick almost literally. Hopefully John hasn't already found someone and left with them, but she gave him a bit of time to get properly tipsy before making her appearance. Where is he?...Oh, there. She flaunts herself and sure enough, he's there with a cheeky pick up line. She can't say yes fast enough.
"What's your name?" he asks.
"Sher...lock." At least half of that is right, and if she gets to make him scream that in orgasm she'll keep half the audio file as a treasure.
She'd happily suck him off in an alley, but he's already saying, "Your place or mine?"
"Yours." Obviously. Is John drunk enough not to notice the truth about her if she angles herself just so? Will he maybe take her from behind, blinded by alcohol and lust, uncaring, and let her masturbate furiously while he's at it? That's too good to hope for, isn't it?
It is. Back in the flat, after a little heated fumbling, it becomes quite glaring not just what Sheryl is, but who she is. No cover of dark or confusing, stroboscopic lights here, and he's not drunk enough to lose every last brain cell. She should have never agreed to follow him, but she couldn't refuse him anything earlier.
John stops abruptly. "What the fuck, Sheryl?" he blurts out.
Experiment. Just say, "Experiment," damn! But her throat doesn't work, words stuck to her epiglottis. The silence stretches ominously.
"Was it all a joke at my expense?"
She shakes her head. No.
She expects him to guess her excuse next, but instead he asks, "If I were well and truly wasted, would you have gone trough with it? If I didn't look further than your ass?"
She nods. Yes. Not good maybe, consent issues raised with people drunk enough not to know what they're fucking, but she owes the truth to him.
"Do you want to have sex with me?" he queries, serious, his voice holding a no-nonsense tone.
She blushes, shrugs, and finally finds words. His words. "You're gay."
"Answer, Sheryl." It's the captain's voice, and Sheryl will never be capable to resist it.
"I wouldn't have entered that place if I didn't want it," she confesses. Let him mock her. Scold her. Whatever.
"You're in luck then, because I find my sexual orientation to be quite oriental these days."
"Uh?" was the most intelligent reaction she could muster. Horrible, insensate puns? What the hell, John?
"You know the yin and yang symbol, right? No matter how much you think you're firmly on one side, there's always a speck of the opposite inside. And I'm finding out that I have one single instance of possible heterosexuality, and there's your name written across it," he boldly admits.
Later, she will ask why he hadn't said a word, and discover that he wasn't going to put pressure on her, make things awkward, possibly ruin their friendship. That he didn't believe she'd be interested. That he thought he wasn't good enough for her. (Ridiculous; it's obviously the opposite. He'll realize that sooner or later.) Later still, he'll admit to have pulled her male impersonation in order to pretend it was her, so things couldn't have been better. Not now, though.
Now, she groans, "Prove it. Make love to me." (Awful slip of tongue. It's just a synonym for what they're discussing, yes, but she shouldn't utter the l- word. It's too reckless. Asking – implying – more than she can ever receive. Attraction is already more than she deserves.)
John doesn't scold her for it, strangely. He smirks, and announces, "That, Miss Holmes, can definitely be arranged."
P.S. If you want to see actual sex let me know. I might be persuaded.