It's called biology. Ten men on an island with 1000 women have a chance at a civilization. 1000 men with 10 women, not so much. Women are biologically more important than males. It's a fact, and any society that fights biology is doomed to failure (in the long term)...
I don't dispute your point historically. However, in the modern Western world, abortion is doing more to reduce the number of kids being born than letting women serve in the military.
The experience of the British abuse of Afrikaner women in South Africa during the Boer Wars is just one relatively recent example of the damage done over the short- and medium-term to a society by targeting the women and children, thereby cutting the reproductive rate. Farther back in history there are many more examples of the need to protect women in general, especially mothers and their children.
I do not agree with Rick Santorum on birth control. However, it's pretty hard to deny the raw facts of reproductive rates that have caused Orthodox Jews to become a powerful force in what was once a largely secular Zionist state of Israel, that have caused Roman Catholics to become an increasingly important segment of the population of historically Protestant societies in Northern Ireland and Scotland, and that threaten to make Western Europe into an Islamic region due to declining birthrates of white Europeans.
Declining birthrates have much less to do with women in the military than with whether women should be stay-at-home mothers with large families. I don't think anyone elected to office on the national level in modern American political life is going to argue that the government should be telling women they have to be stay-at-home mothers. If families want to do that, it's their choice, and government should respect that choice, but it's not government's role to make that choice for women and their husbands.
We're conservatives, and while it's true we're not libertarians, we also are not theocrats. The United States Constitution does not authorize the United States government to impose religious views on its citizens, and a religiously based requirement of large families and stay-at-home motherhood isn't something I would consider to be a proper role for civil government even if America were an explicitly Christian and covenanted nation. Those decisions belong to families, not the civil government.
China.