I've seen pictures like this making the rounds on Reddit recently:
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I just wanted to give a quick explanation of why these are weak arguments, and how to respond to them when you see them.
What they're essentially saying here is that because there are
genetic disorders, there are
birth defects, in which some people do not precisely fall under the biological sex binary that the vast majority of human beings fall under, then that means the whole concept is arbitrary. This is like saying that puberty isn't a natural part of human aging because some people have disorders like Kallmann syndrome where their bodies don't naturally undergo puberty.
And I think it's also worth looking into what some of these exceptions to the biological sex binary they're alluding to actually look like. One of the most common examples is Klinefelter syndrome, in which a person is born with male genitalia but with two X chromosomes. What does this mean? Does this make them androgynous, something that completely defies our society's understanding of gender? Not quite. The only visible symptoms of it are man boobs and an inability to grow facial hair. Also, a smaller-than-average willy. Most people with Klinefelter syndrome don't even know they have it.
That's it. That's the smoking gun they're using to try and deconstruct one of society's oldest and most basic concepts.
I really hate arguments like those in the screencaps I provided because not only are they inane, they're also incredibly dishonest. It's an obfuscation tactic, designed to work on people who don't understand biology (and can therefore immediately call them out on their bullshit) by muddying the waters.
TL;DR: never ask a woman her age, a man his salary, or a trans activist what the word "syndrome" means.