In Spanish, everything is gendered, usually descenable by an -a or -o ending.

So Spanish requires you to pick the male/female linguistic gender to refer to a person in order to say that their gender doesn’t fit on the male/female binary.

I believe Spanish speakers just resolve it by using -o by default, because linguistic gender is not identical to social gender.

It’s roughly like if English made you say “they’re masculine-non-binary”.