The constitution made it impossible to make a less sexist law, because it says that women cannot be forced to military service. It is an old document, and it is based on old role models. Modernizing the constitution would require 2/3 majority, and the government was already struggling with making a law at all.

This is an explanation, not a justification.

In a scenario where you are losing a significant part of the population to war, it's better that it be men.

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