usage - What’s a word for someone who does not believe in sexual orientation?

Asexuals are people who have no sexual desire, not people who have no gender.

People who deny having a gender are quite rare (people normally exist on a continuum, and it's not uncommon to identify as partially both).

People who don't like the idea of gender (which is what you seem to be asking) are just trying to change language and philosophy. Or they are delusional.

Ignore it (well, except maybe in their presence?)