The short answer is: it’s complicated.
The slightly longer answer involves the difference between “queer” and “homosexual.”
Actually, you know what? Let’s talk about that. I think it’s worthwhile.
So, briefly: I think that Sherlock and John are a canonical queer couple. I feel that has been demonstrated as unequivocally as it’s possible to do without them actually making out or something.
They have not, thus far (as I think we’ve all noticed), been demonstrated as an unequivocally homosexual couple.
But queer does not necessarily equate to homosexual.
Ceasing to be brief (Jesus, I should install a read-more in this thing): My canonical read on them at this moment—the thing that feels actually true to me, what I see in the show and think that I can reliably argue and support with evidence as outright being there—is that Sherlock and John are in a committed asexual relationship.
Not just a friendship, although it can be hard to explain to people who haven’t had one what the difference can be. An actual committed asexual relationship.
I want to chime in to introduce the term “pair bond,” which I think is what prettyarbitrary (and BBC Sherlock itself) is describing. “Pair bond” is a term that was originally used in biology to describe animal mating behavior, but is now also used by psychologists to describe a committed and exclusive relationship that both parties are heavily invested in. This is often romantic, sometimes sexual, and sometimes floating between friendship and romance.
The hallmark of pair bonds is that the bonded pair consider their relationship essential to their happiness and identities. Forming a relationship of a similar nature with someone outside the bonded pair is perceived as threatening, resulting in jealousy and separation anxiety.
There’s no doubt in my mind that John and Sherlock are canonically pair bonded. They simply cannot form close romantic relationships with other people, because that would be violating their pair bond. (John continues to try because, to me, he is not sexually attracted to Sherlock and therefore erroneously interprets their relationship as close friendship.) We repeatedly see John and Sherlock threatened by potential romantic partnerships and openly offended that the other would even consider engaging in one.
John doesn’t think he has to choose between Mary and Sherlock, because he is in a sexual and explicitly romantic relationship with Mary, and in a relationship he can only describe as “friendship” with Sherlock. And perhaps those two pair bonds are not in conflict for John. But to Sherlock, it is obvious that the John-Sherlock pair bond cannot be reestablished if John is pair bonded with Mary. The show repeatedly compares John and Mary’s relationship with John and Sherlock’s, not because they are the same, but because they are too similar to coexist- at least as Sherlock sees it.
John and Sherlock don’t need to make out or confess their love to resolve their relationship. Pre-Mary, it was already resolved. After John’s conversation with Irene, they both acknowledged that their relationship isn’t exactly friendship, that they can’t successfully date anyone else, and that whatever they’re doing now is enough. If Moriarty hadn’t separated them, they would have been content to continue that relationship indefinitely- so long as no one else intruded.
So, as prettyarbitrary stated, John and Sherlock are already in a committed asexual relationship: a pair bond. One or both might be sexually attracted to the other, but it isn’t a necessary component of their relationship. Aside from media representation, their sexual orientations are irrelevant. (But media representation is super important and it’s totally justifiable to want to see it in the show.) From this standpoint, there’s nothing more to make canon.
Here is that spectacular meta on pair bonding. This is basically what I was talking about. And in fact it finally gives me a framework that really works to separate the vague concept of ‘asexual attraction’ from sexual attraction in a way that can be articulated. I’M SO EXCITED. What a gift from lunanimal!