This has little to do with human society. Western society, maybe. Plenty of other societies don't assert a distinct binary for gender (which doesn't even exist in sex; intersex people exist at the same rates as people that are red-headed, and you wouldnt call red heads unnatural).

Even if it was "natural" you're making the choice to treat someone like shit. You can backpedal onto what all your peers are doing all you want, but that doesn't change how you as an individual are making another human being, who has done nothing to you, feel