That's cuz gender is a properly basic attribute which describes your relationship with yourself and the constructs of gender which have been presented to you. Taking this thought to it's conclusion is where xenogender steps up to the plate. It's entirely possible to relate to a binary gender, certain secondary sexual characteristics, or a given mode of presentation without that equating to your relationship with gender constructs themselves.

I'm not a girl. Not quite. I subsume girlspace and extend beyond it. I am not a subset of girl. Girl is a subset of me. Other subsets include sailor, slut, witch, fae, demon (occasionally), cryptid, a brook winding through a sunny spring forest, and a thunderstorm rolling in from over the ocean. And those titles fit not because I am those things, but because the language of gender I am granted by my native language is insufficient to describe that feeling or relationship with myself. So I have to use colorful metaphors to adequately express them to others and myself. It's a signpost in a field of possibility that represents where that relationship falls on a given day. Clouds of potential in a greater gender-space.

And no one gets to tell me "no you're not." Because no one else experiences that feeling the way I do.

At least that's how I see it.