Well, you see, if men stay alive, but women are killed, society collapses eventually as not enough new people are born. It sucks being a man in this scenario, but it is what it is.
Arguably, not enough people are being born as it stands. We're already in your collapse scenario.
And if you include women (well, all genders) directly in the war efforts you double the amount of soldiers you have, which would increase your chance of winning and not needing to repopulate.