But those aren't the two options. If loss of life were equal due to equal participation, you'd get 45% loss of each sex (using your 90% of men figure as the potential loss), which (while still devastating) isn't the same argument for losing the breeding capacity of the country.
The reality is that those sorts of rules are very much based on culture tradition ('historic'). It takes a long time for parts of our culture to change. Only a few countries have mandatory service for women (Finland's neighbors Norway and Sweden among them). Israel does as well, but has a large exception on religious grounds.
Also, mandatory service doesn't necessarily mean mandatory combat service, although I don't know how those countries just mentioned manage that.
