Etymology begs to differ.

Recently took a look at the word "Engendered" which is defined as "to make happen", "give birth to". To my shock, it is based on the same french word as Gender with the prefix "in" from latin. The word is Genre , which means "make of", "type of" it comes from Latin Genus. Genus, is the base of Gender, meaning... family.

By the very history of the words, gender cannot be a spectrum, as the nature of it to the fullest extent of our knowledge. As it requires specifically 2 genders to engender a family. Out of 3; Male, Female, Other.

Even when a species reproduces asexually, we use "male" for the parts that give the seed; we use "female" for the parts that house the birth.

We could use any words. But the irrefutable feature of "gives seed" and "houses seed" are the defining traits.