I haven't come across the term "heteronormative" before, but I found this webpage from 2005 which defines it as implying "that standard sexual relationships are only between males and females." Wikipedia have a longer article about it, although I advise caution about taking any of their information at face value.
Leaving aside Harry Potter, this is really a question about stories in general, regardless of the medium. The concern seems to be that if you only ever hear about heterosexual couples then you won't realise that there are alternatives, and this would make it harder for people who are growing up different (e.g. if they're homosexual or bisexual). I can sympathise with this point of view, but I think that there's an important distinction between "All of the characters in this story are straight, as far as we know" and "There's no such thing as gay people, la la la, I can't hear you!"
Taking an analogy from my own experience, I have anosmia, i.e. I don't have a sense of smell. That webpage is due for an overhaul, so I think this is a good time to go into a bit more detail.
Back when I was at primary school, I remember being taught that there were five senses: sight, hearing, touch, taste, and smell. However, there were some people who were blind or deaf; the implication was that everyone could touch/taste/smell. My anosmia is congenital: I was born this way, but I didn't really figure it out until I was about 8 or 9. Since I "knew" that everyone could smell, I automatically assumed that I could too, even though the evidence didn't support this. Sometimes I'd have friends round, and while we were playing in my room they'd say "Ah, your mum's cooking fishfingers for tea", and I'd be amazed at how they could possibly know that, given that there were several solid walls in between. (I think this is why I keep an open mind about the possibility of psychic powers nowadays.)
Anyway, I was able to fake my reactions well enough to keep deluding myself, so nobody else noticed either. The turning point came when we got a "Scratch'n'Sniff" card with the Radio Times one week. This was intended to be the Next Big Thing in interactive television (c. 1983), much like 3-D glasses, and the idea was to scratch off the relevant sections of the card at pre-determined times during a TV program that was being broadcast on Saturday evening, giving you total immersion in the scene. I scratched one of the squares off in advance, but nothing happened when I sniffed it, so I thought "Oh, that one's broken". I then tried another, but by the time I'd got through the entire card I had to consider that there might be another explanation. (In retrospect, I do feel slightly guilty about depriving my family of the chance to use the card as it was intended.)
A few years later I was at boarding school, and I mentioned this to the school doctor (at the infirmary). However, he didn't believe me; after all, it's a well known fact that everyone has a sense of smell, so I was presumably just making up stories as a form of attention seeking. He gave me a bunch of tests to prove this: these involved being blindfolded and then given things to sniff, on the grounds that I wouldn't be able to control my instinctive reaction to a bad smell. When I "passed" these tests, he was a bit stumped, so he referred me elsewhere. My memory's a bit hazy on this point, but I was bounced around to a few different doctors, and each time it was the same: they didn't believe me, and didn't accept the previous doctor's diagnosis, so they repeated the tests themselves.
One key problem was that nobody knew why I couldn't smell. Anosmia is a pretty rare condition, so there hasn't been a great deal of research done on it. In my case, I doubt that it's genetic, unless it's a recessive condition that's been lurking in my family tree for a very long time. (Mind you, I'd like to get that confirmed before I ever have children.) The alternative is some form of environmental damage, e.g. if things went a bit wonky in the womb. (That's the theory that I lean towards.) Finally we made some progress: X-rays revealed that I had a bone blockage in my nose.
At the time my father had private medical care as a perk of his job, and this also applied to his family, so I was able to get an operation done when I was 13 to drill away the blockage (and remove my tongue tie) without having to sit on the NHS waiting list. The frenectomy for the tongue tie worked fine, but the bone drilling was less successful: the excess bone is now gone, but it didn't restore my missing sense of smell, so the only effect is that I now get hayfever in the summer when I used to be immune to it. I was quite disappointed by this result, and I do feel rather ashamed of the way that I spoke to the surgeon afterwards (I pretty much accused him of incompetence).
Since then, I haven't gone any further in search of a "cure". I think my best chance is to get an artificial substitute, i.e. a mechanical sensor that I could carry around to identify odours in the air. This experience did leave me with a rather dim view of the medical profession. As I say, anosmia is pretty rare, so I can understand that lots of people (including doctors) won't have heard of it before, and that's fine. However, I do think that they should have tried to do some basic research before coming to the conclusion that I was lying. It's only really been in the last five years or so that I've been able to put this behind me, and nowadays I happily work with doctors (in my day job and at SJA). However, if I do have a medical problem then I still view doctors as a last resort: I prefer to muddle through on my own if at all possible.
As well as doctors, I also got quite used to other people being surprised when I told them about this. As I mentioned on my webpage, the same questions tended to crop up over and over again. (As a side note, I've heard from a few women with anosmia, and apparently the first question they always get from boyfriends is "Does that mean I could fart in bed and you wouldn't mind?") As I've got older, people have been much more ready to accept my claims at face value; I don't know whether that's because I have more credibility as an adult, or just because there's an increased awareness in society that there are plenty of weird medical conditions out there.
Over the last few years, I've been compiling a list of fictional characters with anosmia. (I'd include real life celebrities too, except that I don't know of any.) So far, this list is pretty short:
* Alan A. Allen (from Thunderpants).
* Odo (from Star Trek: Deep Space Nine), although he gained a sense of smell later in the series.
* Selma (from The Simpsons).
Odo's anosmia was only referred to in a couple of episodes, and I think that Selma's was only referred to once, so you have to pay quite close attention to pick up on it. Would you have known that trivia fact about those characters if I hadn't told you?
In most stories, the characters are either explicitly referred to as having a sense of smell, or it's not mentioned at all. For instance, in chapter 30 of "Harry Potter and the Order of the Phoenix" it says that "Dungbombs and Stink Pellets were dropped so frequently in the corridors that it became the new fashion for students to perform Bubble-Head Charms on themselves before leaving lessons, which ensured them a supply of fresh air, even though it gave them all the peculiar appearance of wearing upside-down goldfish bowls on their heads."
Then in chapter 9 of "Harry Potter and the Half-Blood Prince", the Potions class includes some cauldrons full of Amortentia. Quoting from Hermione, "it's supposed to smell differently to each of us, according to what attracts us". The book says that "Harry, Ron and Hermione sniffed interestedly as they passed large, bubbling cauldrons", implying that they could all smell something, and it's explicit about what Harry and Hermione could smell.
So, should there have been an anosmic character in the Harry Potter books? Maybe just a minor one? I'd certainly have been happy to see one, in the interest of raising awareness (given how popular the books are). If I'd known that anosmia existed when I was young then it would have been easier for me to figure out that I was different; similarly, if other people had known about it then I wouldn't have encountered so much scepticism.
I can also think of ways that anosmia could benefit the story. For instance, I recall a particular situation from my second year at CH, when one of the other boys let off a stink bomb in my cubicle. It didn't really have the desired effect, because I didn't notice it, so I just sat there reading. However, when the smell spread to the neighbouring cubicles, those boys were then a bit miffed with the boy who'd set it off, so his plan backfired.
Quoting from chapter 19 of "Harry Potter and the Goblet of Fire": [Harry] and Hermione spent a long time going over plans for forcing any stragglers out of the common room on the night in question. If the worst came to the worst, they were going to drop a bag of Dungbombs, but they hoped they wouldn't have to resort to that - Filch would skin them alive."
Suppose that Colin Creevey had anosmia: Dungbombs wouldn't have bothered him, and he'd probably relish the chance to spend some time alone with Harry, so this would have been an extra obstacle for Harry to overcome.
So, as I say, I'd have been happy to see an anosmic character. I don't know why there wasn't one; it's possible that Ms Rowling made this choice deliberately because she hates anosmics, but I think it's far more likely that the idea never occured to her. After all, this condition is unusual, so it's normal to be able to smell.
I could go off on a rant about "osmonormativity", railing against the novels for this criminal oversight and the way that "my people" have been consigned to the ghetto of literature (bruv). After all, the best known reference to anosmia is a joke, which cruelly mocks the afflicted:
"My dog has no nose."
"How does it smell?"
However, I'm not going to do that, because I have some sense of perspective; this issue may be important to me, but I don't expect it to be a high priority for everyone else, and I'm not going to start inventing reasons to feel insulted.
I'm sure that there are various stories which could be adapted to be more representative of modern day society. However, the problem with introducing token characters is that you wind up with something like Extreme Ghostbusters. The writers have claimed that they didn't deliberately aim to be PC; for instance, they chose to put Garrett in a wheelchair because he seemed like the stereotypical "jock" character before that. However, it does come across as being an exercise in quotas: "The black guy! The hispanic guy! The guy in a wheelchair! The woman!" (Having said that, I quite enjoyed the series.)
If I started putting anosmic characters into all of my own stories then I'm sure that people would (and should) criticise this as self-indulgent behaviour. The closest I've come is with my short story Psilence (which acts as a metaphor). I'll only use that character trait if I think it contributes to the story, and I won't ask for more than that from other authors.
So, to conclude, I think it's unreasonable to seek examples of your chosen demographic group in every story you read; I prefer to judge each work of fiction on its own merits. However, I should just clarify that I'm not trying to single out the post that I linked to at the start for specific criticism. There are several other people with the same attitude, and that's just the first one I came across; if you read that then you'll be able to hear the opposing point of view.