Because they've asked not to be.

I knew someone born by the name Todd. He hated the name, and changed it to John. Anyone who knew he wanted to be called John and still called him Todd got an earful - because he asked not to be.

I'm not talking about names, I'm talking about being identified as a man or a woman.

If there is no definition of what a woman is, then being identified incorrectly can't exactly be condemned.

Again: They asked not to be.

Using an identifying word, that you've asked not to use, is a dick move. Accidents can happen, until you get used to something, and most people are fine with that and will give a gentle reminder. But continuing to use a term you've been asked not to, isn't fine.

Let's put it another way.

There's a girl I know, who was born Tiwi. However, though she looks Tiwi, she asks not to be referred to that way, and prefers Australian or native, or indigenous. For why? Every single one of the Tiwi Elders raped her, at one time or another. She shouldn't need to explain that horrifying background for you, reliving those oh-so-pleasant memories, for you to see reason in not calling her Tiwi. Because she's asked not to be.

Similarly, people have asked to be identified a certain way. Some of the reasons may be traumatic. They shouldn't have to relive that trauma, just for you to consider acquiescing to their effortless request.

All that should matter... Is they asked not to be. Fin.

But there is a definition: female mammals are XX and have female morphology. So you're not helping the conversation by investigating consequences of a false premise.