Sherlock is ten when the neighbours' kid asks him what's wrong with his brother. He feels insulted on Mycroft's behalf; nothing could ever be wrong about his big brother, he's the smartest person Sherlock knows, and his role model too.
"Ben says he saw him kissing a boy. Now that's disgusting."
"You're disgusting," Sherlock seethes, kicking the moron on his shin.
When he gets home with a black eye and a split lip his brother raises a questioning eyebrow, then sneaks him into his room without their parents even noticing. Mycroft's hands are gentle as he tends to his wounds, and Sherlock finds the courage to ask the question that's dancing on the tip of his tongue.
"Boys are supposed to like girls, aren't they?"
"There's no such thing as 'supposed to', Sherlock," his brother explains patiently. "Some boys like girls, while others like boys better. And there are some who like both girls and boys alike."
"Why?"
"Dunno. That's just the way it is."
He suspects that Mycroft actually knows – he knows everything after all – but it's probably one of those things only grown-ups are entitled to talk about.
"Is it okay to like boys then? Tim says it's disgusting."
"Tim's just a kid, and a dull one too. Is that how you got yourself beaten up?"
"He said some horrible things about you. He said –"
Mycroft shakes his head. "Let him talk. People do very little else."
"How is it like – kissing someone, I mean?"
"Satisfactory. It's the endorphins, you know."
"What are endorphins?"
His brother spends the rest of the afternoon teaching him everything he needs to know about neurotransmitters and the chemistry that makes them work. Sherlock drinks in all the fascinating information, and the incident with the neighbours' kid is quickly forgotten.
xxx
He's seventeen when he starts wondering what's wrong with himself. His classmates are always boasting about their latest conquest, while he's only interested in chemistry books and detective stories.
In the end he decides to give it a try, treating the whole thing as he would do with one of his experiments. Girls are probably the easiest place to start, there's one that has been pestering him to go out with her for the better part of the spring term.
Sherlock is not impressed in the slightest when she threads her fingers through his hair, nor when she meets his lips for what he presumes would constitute a kiss. He tries to recall everything he knows about the chemistry of love, but fails miserably.
This is just stupid; and wrong, so very wrong. He catches her wrist before her hand can reach its intended destination, pulls away from her embrace.
"I'm sorry," he states flatly, ignoring the flash of disappointment in her eyes.
Girls aren't really his area, that much is apparent; he supposes that means he's gay, just like his brother.
It doesn't take him long to find a suitable subject to test his theory on. Victor is a nice guy, and he seems quite delighted when Sherlock puts on a show of being interested.
His lips are more demanding than Violet's, and yet he finds himself just as unresponsive to the other's touch as on that previous occasion. This isn't working, he has no idea how it's supposed to work in the first place.
He feels like an idiot when he excuses himself and hastily walks away. That night he lies awake in his bed, thinking back to the conversation he had with his brother some seven years before.
Mycroft notices there's something off about him when he gets home for the weekend, promptly demands to have a word with his little brother.
"You can't order me around, Myc. I'm not a child anymore."
"Apparently not. You're still sulking just like a kid would do."
He slumps into his brother's favourite chair, turns a weary look out of the window. "You were wrong, you know."
"I beg your pardon?"
"You said that one can like either boys or girls, or both. Well, I don't."
They stare at each other for a moment, then Mycroft slowly shakes his head. "You were ten, Sherlock. I wasn't particularly keen on discussing the whole spectrum of human sexuality with a brat."
He pauses, biting at his lower lip in a vain attempt to conceal his vulnerability. "Are you now?"
"There's nothing wrong with the lack of sexual attraction. It just means you're asexual."
"People don't tend to see it that way."
His brother sighs. "Ignorant people, maybe. You are what you are, Sherlock; no one else is entitled to tell you otherwise."
Silence stretches between them, until he's the one who eventually breaks it. "Thank you, I guess?"
"You're welcome, brother mine," Mycroft smirks, and he almost thinks he misses the old camaraderie they used to share when they were kids.