First, the level of commitment we’re talking—a lifelong, marriage-style ‘melding our futures and making choices based on the assumption that you will still be here’ kind of commitment.
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.
Because admittedly, asexuality gets even less media representation than homosexuality. But then again, it’s a very subtle thing, in some ways. A very internal thing, not easy to render in a visual medium.
*massages temples* How can John be asexual when he put a baby in Mary.
Hee! It’s okay! I’m not saying John is asexual. I’m saying his attraction to Sherlock may be asexual.
Just because a person can experience sexual attraction doesn’t mean they can’t also experience asexual attraction. In fact I am pretty sure there are lots of sexually active and engaged people who have had asexual attractions in their lives.
For example, anybody who has a spouse or life partner is probably not only sexually attracted to them, are they? They’re usually also romantically attracted. (In fact, sometimes they’re not sexually attracted but they are romantically attracted!)
John is clearly not an asexual person. He definitely experiences sexual attraction towards women. Since he’s married Mary, probably he also feels romantic attraction for her. And additionally, based on what we’ve seen, he feels some strong asexual form of attraction for certain men, like Sholto and Sherlock. This particular form of attraction might be romantic. It might be something else. It’s hard to say.
(Whether he can also feel sexual attraction towards men, we don’t know. Though he does keep claiming to be “not gay,” so he seems to think not.)
In a minute, I’m going to reblog another reply I had about the concept of “pair bonding.” I think it really nails this subject, and clarifies how romantic attraction and sexual attraction separate out from one another.