"The epithets of drill instructors or fellow soldiers - 'maggot,' 'faggot,' 'snuffy,' 'pussy,' or simply 'woman' - left no doubt that not becoming a soldier meant not being a man."

 - Mark Gerzon

This episodes question is straightforward: why have human societies across cultures, continents, and eras have met the challenge of war in basically the same way: by assembling groups of fighters who were usually exclusively males.  The answer I would argue is also straightforward.  We just happen to have forgotten it because our societies and our wars do not resemble anything in the prior 300,000 year history of Homo sapiens.