Saying that man and woman are the only genders is actually LESS nuanced than saying that earth, water, air, and fire are the only elements.
This is fantastic.
This is important to me as someone from a science background, because a theoretical model should explain ALL the evidence, not most of it and ignore the outliers. If your model of gender doesn’t account for nonbinary people, and your model of sex doesn’t account for intersex people, you need to change the model, not the people.