And this is why Heinlein was an engineer, not a historian.
No society prior to Christianity, except the Jews, had "women and children first". Children and women are cheap, you can make more of them, and if they're not up to your standards you can kill them. That's why killing newborns just for being female was practiced from Italy to China and from Switzerland to Zululand. It still is in all the parts of the world that didn't go Christian at least for a while.