Binette is giving you the lecture that happens in every first year college bio class to students.

Basically evolution has in the past and could in the future (as soon as tomorrow) add a third sex to humans if there was evolutionary pressure to (which there may be). Physics is only deterministic in the short term.

The other issue is that when people (especially those in any scientific community, such as biologists) use the word gender, they specifically mean the list of attributes different societies place on biological sex characteristics they can observe.

Gametes are not something an unaided eye can identify in society so its not useful to assign gender - which no matter how you define it will never line up perfectly with biological sex due to environmental factors. Might as well use the SRY gene or even “the presence of sufficiently SRY receptors”. This is why in society we largely determine gender by what biologists call “secondary sex characteristics” aka ones not actually required for reproduction.

If assigning a gender was evolutionarily important, we’d be assigning it based on primary characteristics like you’re suggesting. But that didn’t happen. The fact is isn’t may suggest its an evolutionary disadvantage to do so.