Please stop pretending it’s unrealistic for a work to have no LGBT characters. Humanity is 96% straight. The remaining 4% is half gay and half bi. Statistically, half the bi people can be assumed to be in an opposite-sex relationship at any given time. So there is a 3% chance of any character being in a same-sex relationship—so if there are fewer than 33 characters, it is not unrealistic that none of them are.
Of course, fiction doesn’t have to be realistic, and protagonists are not statistically typical people. You can have whatever demographics you want for them—you can write stories about albinos, who are a lot less than 3% of the population. But other creators can have whatever demographics they want; you can have all the representation you like, even overrepresentation, but others have the same freedom. Even including underrepresentation.
Nobody is actually required to represent any group, even if statistically a representative would likely show up; you can absolutely have a work set in America with no major Christian characters at all, even though Christians make up 13 Americans in every 20. Nobody has to have LGBT characters even if they have so many characters that a same-sex relationship is statistically likely to happen among them. (And not only because portraying a gay couple in a manner that doesn’t exactly correspond to some Twitter troll’s wholly arbitrary prescriptions, known only to them, is asking to be harassed. Though that is a factor.)