There probably are not going to be any more wars like WW-II with 3000 mile fronts, nor any real need for countries like Germany or Russia with the kinds of weapons they have to need 5,000,000 people carrying rifles at one time. There is no real reason why a country with 100,000,000 people needs half of those people to be men. If you were to artificially skew sex ratios to something like 1 - 6 or 1 - 8 instead of the natural (for humans) 1 - 1, then a birth rate of 1.3 such as Germany has had recently would be more than enough to keep population levels together.
Are you suggesting a polygomous society? Or that less men in the world mean less wars? (because it doesn't seem to be working that way in the ME!)