Here's one possible reason for people giving a pass to biologically and demographically impossible societies in fiction, they don't think across generations. We humans think within our lifetimes and many times fail to even do that.This is probably why talk about scale and sustainability (and the problems of mass immigration, the loss of traditional sexual mores, and the rise of unmarried mothers) will go over the heads of most people.
To imagine a future one, two or three generations down the line is not likely to happen. We've seen it play out in history. The whole time the western Roman empire was failing the Romans themselves couldn't quite put their fingers on why it was failing to a degree that would stop much less retard that failure. They were captives to choices made in a past not of their making therefore not in their minds.
Vox has discussion threads on both blogs about the necessity of "sexism" (that is, a realistic recognition of sex differences) in fiction: "Sexism" is a literary necessity and The need for "sexism" in literature.