the viewer is completely free to imagine that homosexuality exists and is accepted in human society.
Be really nice if that was backed up by something
But it does not have to be backed up by something. Canon is silent on it so the issue is fair game for the viewer to fill in at will.
A viewer could, as you seem to, imagine that homosexuality is gone from humanity. How such an situation would be feasible, I do not know, but since canon is silent on the matter one can choose that interpretation.