I have no problem saying that 'gender is a spectrum', unlike many posters on here - see, we're not a complete echo chamber!
'Gender' when used to mean the outward manifestations e.g. dress, mannerisms, etc., makes sense to me, but I know it is a social construction. What is 'male dress' or 'female mannerisms' vary from culture to culture and from century to century.
I've enjoyed being gender-non-conforming all my life, and as soon as I was politically aware, I was a radical lesbian feminist who fought against the imposition of socially constructed gender stereotypes. In particular, influenced by my own childhood, I think it's so important to avoid forcing gender stereotypes on children, so boys and girls are children first, no child is born in the wrong body, and boys and girls should be allowed do what they damn well like in terms of toys, dress, etc.
None of that in any way inhibits my ability to understand that gender is not sex, and that human sex is binary and you can't change from one sex to another. So clearly sex is not a spectrum.
By separating 'sex' and 'gender', I have no difficulty in seeing one as a spectrum and the other not.
Switching the astronaut/flat-earther simile around doesn't work, as the observable [and observed from space!] scientific facts are on the side of the astronaut, not the flat-earthers; in the 'Is sex a spectrum?' debate, science is on the GC side.
Thank you for giving us an update of Disraeli's famous response to Darwin, we can now say 'I am on the side of the astronaut'