Radical gender theory does in fact make the bold claim that biological sex isn’t real. That’s right, sex, not gender. The postmodernist underpinnings used to support these types of ideas are rooted in concepts like “objective truth does not exist” and “everything is a social construct”.
From the article:
"The biology of sex is real, but it’s extremely complicated, and there is sometimes no easy way to draw a line between the biologically male and female."
Having now read the top 5 articles for that search term, I still haven't found one that makes the claim your suggesting.
I'm sure if I dig hard enough I could find that someone has said it non-ironically, but if I dig hard enough I can find someone saying the moon landing didn't happen or talking about lizard people non-ironically. That doesn't make it a mainstream movement.
One recent event that brings it to light is JK Rowling said something along those lines, basically that women menstrate¹, and that caused a lot of controversy. I'm sure you'll be able to find plenty of articles against her for saying that.
[1] https://twitter.com/jk_rowling/status/1269382518362509313
It's good you haven't understood people saying sex isn't biological, because it's not a defensible position. Hopefully their voices don't become more prominent.
Lots of women don't menstruate. Children, older women, women who've had hysterectomies, some women with intersex conditions, and transwomen.
Some men do menstruate. Transmen, and some people with intersex conditions.
This argument is not saying that "sex isn't real". It's saying that using terms that define sex are not good enough for some topics.
Also, on your other points: Objective truths can exist, but not all measurements are objective. Not everything is a social construct, but the things that humans believe without empirical evidence are all social constructs.
Postmodernism isn't hard. Y'all're just extremely convinced that reality is locally real, just because you can sense it so vividly, and you're blind to your biases as a result.
No.
Transphobes[1] will say that sex is obvious and the binary is clear.
The argument being made against transphobes is that definitions for "female" (or "male") can't exclude trans-people while including everyone else traditionally thought of as female (or male). That's something that transphobes claims is easy, and they're wrong.
[1] Many transphobes say this. It doesn't mean that anyone who says this is transphobic.