Is literally any discussion or debate on the topic inappropriate?
Gender is appropriate for sociology. Biology doesn't give a shit what you identify as. It has no place in a biology textbook. It's not moving the field forward, it's trying to push a worse and irrelevant definition.
I disagree that the only way to determine the sex of an individual is gamete size
Bully for you, but your opinion is irrelevant to the scientific consensus.
The author also wrote an article that is addressing your exact questions: https://www.realityslaststand.com/p/how-our-shoes-can-help-explain-the
The article basically consists of “I don’t like it when people do this
Again, this is not just some random opinion. This is is not equal to your opinion. This is a PhD in evolutionary biology writing about the scientific consensus. You're free to disagree with the scientific consensus, but you should admit you're no better than a creationist spouting off "god did it".
You're confusing sex and genotype. Those are variations within a sex. They'll still produce one of two gamete sizes