Scenario A: 100 women, 100 men. 100 men go off to war, 10 men return. But with polygamy, 100 women have 100 children for the next generation.
Scenario B: 100 women, 100 men. 50 women and 50 men go off to war, 5 men and 5 women return. You have 55 women left in the population and 55 children for the next generation.
The limiting factor in human population growth is the number of women that can have children. The cold calculus might be distasteful, but it is there.
by adding polygamy into the mix, you're moving the goalposts of what is really just an intellectual exercise here. The reality is that these sorts of laws are put in place based on what the lawmakers feel is culturally acceptable (tradition). Pretty confident the people arguing for male-only conscription aren't putting much weight on polygamy in their thinking. What they're really thinking is that they need male soldiers to win their war or defend their country...
It's not even official polygamy, it's just people being people and life finding a way. Lots of single mother households raising a bunch of kids. The balance is mostly restored in the next generation.
The cold calculus is also hopelessly outdated in so many ways given the timescales of modern wars and modern transportation speed and connection. Even in an extended war the reproductive capacity is frankly irrelevant as even 1 million moms can't pop out a fully trained soldier. And it isn't a matter of cannonfodder anyway. Gender ratios and population growth may be resolved better by immigration than enforced by isolation polygamy.
It's a good thing then we don't have any country with just 200 people in them.
It's same excuse used when someone asks why safety of women comes before men, conveniently ignoring that the scenario itself is so far out, it's not going to happen.
Scenario B: 100 women, 100 men. 50 women and 50 men go off to war, 5 men and 5 women return. You have 55 women left in the population and 55 children for the next generation.
The limiting factor in human population growth is the number of women that can have children. The cold calculus might be distasteful, but it is there.