Because the original Sherlock Holmes reads as asexual and because John subtly puts down the cultural ideal of romance being more important than friendship every time he loses a girlfriend because he needs to go with Sherlock for a case. Feel free to message me if you have any questions or quibbles.
John chose not to address Sherlock, who had cracked into John's computer again despite his own being in the room, as he went to tend to the tea kettle. He left it on the stovetop to boil after checking the insides for experiments (apparently clean, though John washed it twice to make sure) before looking for a pair of mugs (under the sink), the milk, spoons (in the microwave), the sugar (on the fridge) and the box of Whittard's Assam teabags (still in the cabinet, thank you Sherlock). Since none of it seemed biohazardous, he decided to go for it and put it all on the part of the kitchen table designated for eating.
"John," Sherlock called. "Come here a moment."
John looked longingly at the not-boiling kettle before walking into the sitting room. "What is it?"
"You've got something to ask me, but you're not sure how I'm going to react to it." John sighed heavily, used to Sherlock understanding what he was thinking sometimes even before he did.
"And how did you figure it out this time?" asked John.
Sherlock looked up from the computer. "You're making tea in the kettle, more than enough for you because otherwise it would be boiling by now. You were just muttering about looking for the sugar, but you don't take sugar and you usually tell me to go find it myself when I move it out of the cabinet. You're making tea for me. You didn't bang anything, your tremor is gone, you're keyed up. Nervous. You want me to be in an agreeable mood so that I don't lash out over what you want to ask.
But what do you want to ask about? What sensitive events have happened recently? The last case was dull, even by your standards, the stolen pedigree puppies. The case before wasn't much more interesting, the ex-girlfriend poisoned the tea. You're used to my feelings about murders. What else could there be? Ah, the Woman. Irene Adler. What do you want to ask me about her, though? She's dead, you know that. And I'll take that tea, two sugars."
"No sugar unless you eat a biscuit. You know, you need actual food sometimes," John replied before going back into the kitchen on autopilot. Just as he stepped inside, the kettle began to whistle. He poured the boiling water into the waiting mugs, putting in the teabags. He found the biscuits, which should be safe given that they were still in their tin despite being next to an experiment. He put two on a clean-looking plate and brought them to Sherlock before going back to the steeping tea.
When John was settled once more in the sitting room, tea steeped and milk added (as well as Sherlock's sugar) Sherlock asked him again. "So what did you want to ask me about the Woman?"
John took a deep draw of his tea before standing up and walking towards Sherlock. "You were-you were-she-I mean..." John took a deep breath. "You were interested in her, weren't you?"
"Her mind was interesting, yes." Sherlock looked up, puzzled. "What do you mean, John."
"Sexually. You found her sexually interesting, didn't you, Sherlock?" John bit out, half angry and half embarrassed. He took another drink of tea from the mug in his hand.
"Ah." Sherlock looked back down at the computer, relaxing. "No. Boring."
John blinked several times in quick succession. "What? But you were-the way the two of you flirted-"
"Ah, that. There was more to be gained by going along with that then by not. I'm sure she's aesthetically pretty, but the body is only transport. Sex does nothing for me." Sherlock paused his typing briefly before resuming in a flurry. "I would have thought that by now that would be obvious to you. You've been around me for a while. You are my friend, after all."
"Well, of course you're my friend!" John replied heatedly. "You'd have to be, or I wouldn't contend with your experiments when they get in the way of making proper tea. But even so, it sounded like you were interested in her."
"In her mind, John, how many times do I have to repeat this?" Sherlock pursed his lips. "I thought we got over this the day we met."
John thought back to that day, the one that had changed his life. He remembered meeting Angelo, telling Sherlock that real people didn't have archenemies. Sherlock inviting him to deduce, which was really inviting him into his world. And then him listing things normal people had, ending with girlfriends and boyfriends. And Sherlock replying with "dull". Then when Sherlock came at him with the spiel about being married to his work, John told him that "it was all fine" and Sherlock said "good" and then "thank you".
He hadn't really thought about it, just assumed that Sherlock was picky and didn't want to deal with a "dull" hanger on. But Sherlock kept him around even though there usually wasn't much he could contribute that Sherlock couldn't. He was Sherlock's friend, who were few but not as few as Sherlock would have people think. He might even be Sherlock's best friend.
Still, Sherlock didn't date, didn't go out and have one night stands (as far as he knew, anyway, but the thought was laughable) and seemed uncomfortable with both the institutions of romance and sex except as they could benefit people in a business sense, like with Irene Adler. He tried to imagine Sherlock on a date, but he didn't seem to relate to the institution of romance at all. The night at the circus was proof enough, as were his actions around John's dates.
For all of their time spent together he hadn't put together that Sherlock was an aromatic asexual. Or something like that, he'd better ask before assuming.
Sherlock would no doubt have researched sexuality at one point, as he once did but more in depth, given its relevance to solving murders. Crimes of passion were fairly common, after all, and not everyone was interested in pairing up with someone of the opposite gender, going on dates and having sex and eventually getting married and having kids. He'd seen the fruits of this research on some of their cases, even.
It was a very personal subject, sexualities, and no doubt Sherlock would have come across terms that fit him. Looking back on that conversation, that last "thank you" was too personal, too vulnerable to be a detached approval for the LGBT+ movement. In that moment John had told Sherlock that he'd still be his friend no matter what he was in terms of amatonormativity and sexuality without actually saying it. That was something he'd stick by, for sure, even if he hadn't expected things to happen quite the way they did.
"Oh," was all John said aloud. He smiled, just a little and said, "Yes, I suppose we did."