I tend to think of gender as just "a first approximation of how I want others to see me"
I know this is not exactly right for others but it seems right for me. Gender isn't really just a word, when you call someone a "man" or a "woman" that comes with a bunch of social baggage and expectations that make some people happy and some others unhappy
But at the same time it's obviously not rigid, and there are a lot of differences from one person to the next even if they share the same gender. That's why I say "first approximation". You COULD have as many genders as there are people on the planet, and some people do want that level of differentiation, and that's fine! But others don't, and it makes them happy to fall into that broad category
Anyways that's why my gender is cat