[voyou]

"I never understood how e.g. Spanish speakers think that a door is female or a clock is male. I mean, it's not like there are any body parts you can examine for a definitive answer..."

But "gender," as a term in grammar, just means an arbitrary classification of words for grammatical purposes. It's only a few languages which, absurdly, map these grammatical tags to biological or cultural sex distinctions. English compounds the absurdity by retaining this grammatical distinction only in this one bizarre case.

(My favourite example of the arbitrary nature of grammatical gender is Dutch. Dutch has two genders, common and neuter, which, if you wanted to map them to sex, would mean "either male or female" and "neither male nor female", respectively.)