I haven't read the books or even seen beyond the first movie when it was at the cinema, but the complaint would still only make sense if all other staff were "openly straight".
Effectively, there could be other staff with no hint towards their sexuality whatsoever. They're assumed to be straight, but that's not in the writer's words it's in the reader's imagination. If there's someone with no hints towards their sexuality at all then that's the reader's fault for ascribing "no hints" to mean "straight by default" and not something the writer did.
You can see parallels in the discussion of whether anime characters are "white" or not.
If I draw a stick figure, most Americans will assume that it is a white man. Because to them that is the Default Human Being. For them to think it is a woman I have to add a dress or long hair; for Asian, I have to add slanted eyes; for black, I add kinky hair or brown skin. Etc.
The Other has to be marked. If there are no stereotyped markings of otherness, then white is assumed.
Americans apply this thinking to Japanese drawings. But to the Japanese the Default Human Being is Japanese! So they feel no need to make their characters “look Asian”. They just have to make them look like people and everyone in Japan will assume they are Japanese – no matter how improbable their physical appearance.
You see the same thing in America: After all, why do people think Marge Simpson is white? Look at her skin: it is yellow. Look at her hair: it is a blue Afro. But the Default Human Being thing is so strong that lacking other clear, stereotyped signs of being either black or Asian she defaults to white.
So if Dumbledore isn't explicitly
labelled as gay, he's "assumed straight". But this is itself a problematic attitude, perhaps more so than JKR's thing "btw Dumbledore was gay". And that is in fact a valuable thing that she did it that way because it opens a debate about how gay people should be portrayed, and the cultural assumption that people are automatically straight unless we give them the "gay signs".
(In fact, in the anime thing, asians aren't drawn with "slanty eyes" because that's a western stereotype about asians, not a belief they hold about themselves. In anime, the "other" is the American, and they're marked with cliches Japanese hold about Americans, so that Japanese people will realize they're Americans: big noses, blue eyes, blonde hair, and they talk a specific funny way to make it more obvious. Similar to racial-labeling in The Simpsons, it needs to be all these tropes at once so that locals will realize it's an American and not just someone with a big nose, bleached hair, or funny-colored eyes).