The sort of people who think of sex as a spectrum don't want to accurately describe it at all. They want to make it as muddy as possible.
I want to expand upon this a little bit, and this is going to involve a tangent into something unrelated, so anyone can feel free to skip this if they don't enjoy me geeking out over my own pet interests.
I'm going to talk a bit about temperature. Temperature is a concept that kids get introduced to very early on: it's a measure of how hot or cold something is. Not very technical and certainly not complete, but good enough for kids, because they directly experience sensations of heat and cold, and the idea of measuring that makes sense. Get a bit further along in your education, and you might be taught that temperature is a measure of average kinetic energy of atoms in a material. Now we're getting technical. Now we have what looks like a scientific definition. And around this time, you might learn about things like absolute temperature, and that even though the Fahrenheit and Celsius scales go negative, absolute temperature can only go to zero.
Except it's wrong. That's NOT the actual scientific definition of temperature. It's a
workable definition in most cases (and you can even find scientific sources using such a definition), but there are exceptions to it. And those exceptions can lead to things like negative absolute temperatures, which make no sense under that simplified definition. I'm not going to do a full-blown definition here of what temperature actually is, but the actual, rigorous definition of temperature involves the derivative of entropy with respect to energy. And entropy itself isn't an easy concept to wrap one's head around, and most people never really learn about. So temperature is actually more complex than it looks when you first introduce kids to it. But even though it's complex, even though we generally dumb it down, the full rigorous treatment is not only
possible, it's routinely done. I've done it. I have literally taught college level thermodynamics, where you define exactly what you mean by temperature in a mathematical sense, and in a way that covers all those weird edge cases like negative absolute temperatures.
OK, so what does this have to do with sex? Well, on the surface it might look like the concept of sex is like the concept of temperature. We learn a simplified version (male vs female), and for most of our lives those simple definitions work fine. But then find out that there are all sorts of complexities (how sex develops, how different species handle sexual reproduction, what happens if the developmental pathways go off the rails, etc.) that are harder to fit in a simple "boys have a penis, girls have a vagina" concept of sex. And the "sex is a spectrum" folks play up this complexity
as if it were a situation like temperature, where we're taught this wrong version (sex is binary) for simplicity but the truth is that sex isn't binary at all.
Except... they never get to the full story. They only get to the "sex is more complex than that" stage. They never actually define what sex is, they never demonstrate that this definition produces a spectrum, they never show that the spectrum produces a bimodal distribution. They always stop at the stage of "that simplified definition is wrong". Why can't they get past that step? Because they don't actually
have a definition of sex which produces a spectrum, that says that transwomen aren't males. They want to make people think that the simple "sex is binary" definition fails, just like the simple definition of temperature can't handle negative absolute temperatures. But their own "sex is a spectrum" claim fails, because (unlike temperature) they can never actually define what it even is.