"making the whole world LGBTQA is unrealistic" or why I find this common writing wisdom to be false

A rule of thumb for writing LGBTQA characters or fiction whether you write romance, literary fiction or fan fiction seems to be you can't make the whole world queer.

One or two queer people is great. That's why we are here after all. The main couple will be queer, maybe some close friends but more than that and it becomes unrealistic. The whole world after all isn't LGBTQA.

I've seen making an entire cast of characters, or the majority of characters queer described as utopian, unrealistic, anno...