Imagine there was a society in which blue eyed people are referred to with blee/bler pronouns, and green eyed people are referred to with glee/gler pronouns, and one day someone from that society saw a brown eyed person and had no idea whether to categorise them as blee/bler or glee/gler
There are plenty of examples of languages with two, three, or no grammatical genders (which don’t always correlate with biological genders—e.g., some languages see ”animate” and “inanimate” as genders.)
If it isn’t arbitrary, why is there so much variation?
Edit: I’m not saying that adding information to nouns by splitting them into rough categories isn’t useful, just that the specific dimension on which we make the split (sex vs animacy vs eye color) is arbitrary.
I think it is, just not fully. You’d be hard pressed to find a language with 6 genders, for example.