“The most important by far though is that men are expendable.”
I don’t believe men are “expendable”, or that they’re any more expendable than women. I do think that men are physically superior and incapable of getting pregnant, so it makes more sense to draft men than women. Any woman who is drafted and doesn’t want to serve in any capacity will get pregnant within eight weeks of boot camp.
Each woman is one less womb, one less mother, that many fewer children. Women of child-bearing age are the ultimate limited resource, the bottleneck.
Women and kids are why Horatius held the bridge - whatever the poem says about the ashes of his fathers and the temples of his Gods.