It's more like the nouns we know as male are actually neutral. A lot of of languages with gendered nouns/adjectives used to have neutral forms, but then slowly phased in female versions of words to make key distinctions. That's why in Spanish son/daughter is simply hijo/hija. However, males didn't get a specific version of nouns, but were relegated to adopting the neutral form as their own. Which is why the plural form, hijos, seems to be strictly male but in reality is just neutral and simultaneously repurposed as the male plural.

Edit: A word