Karen Campbell, at her blog ThatMom, did a series of podcasts on "militant fecundity" back in 2008. You can listen to them here: http://thatmom.com/podcasts/militant-fecundity-vs-children-as-a-blessing-series/
As best I understand this phrase, it refers to a teaching that it is God's will that Christian married couples should have as many children as possible. But it goes beyond seeing children as God's blessings, which the Bible clearly teaches (Psalm 127:3).
The teaching is tied to 'culture war', especially if plugged into a post-millennial view of Christ's return. “It is the duty of Christians to bear large families full of godly seed to populate the earth and bring forth what God intended us to have, particularly in America,” Cynthia Kunsman, a writer and blogger who specializes in spiritual-abuse issues, described the worldview at a 2008 conference at Midwestern Baptist Theological Seminary. “That’s how we’re going to get our Christian America.”https://www.baptiststandard.com/news/faith-culture/16187-southern-baptist-attitudes-changing-on-birth-control
I see great mistakes in the idea of militant fecundity.
Genesis 1:28 shows God saying to Adam and Eve, "Be fruitful, increase in number, and fill the earth." God certainly didn't expect Adam and Eve to fill the earth all by themselves, but He created the biological method by which kind reproduces kind. In addition, human lifespan prior to the Flood was extraordinarily long.
Adam lived 900 years, but then after the Flood human lifespan began to drop precipitously. There is a good article about this at Answer in Genesis: https://answersingenesis.org/bible-timeline/genealogy/did-adam-and-noah-really-live-over-900-years/ So, while Genesis 1:28 gives God's general directive for child-bearing, the natural circumstances in which child-bearing happens have changed dramatically since Noah's time. Genesis 1:28 doesn't dictate how many children to have, simply to go have them, as God enables.
What about God's promises to give abundant children to Israel? God's Deuteronomic promises were God's covenantal contract with Israel. God promised the Jews great wealth, vibrant health, military superiority, and abundant children, on condition of faith in the Lord and obedience to the Law. But the Deuteronomic blessings were designed for those who dwelt in the land of Canaan (which is shown by God's promises of victory over invaders of the land). In addition, the New Testament says that Christians are not under the covenant of Mosaic Law. The Deuteronomic dispensation ended at the cross of Christ.
The New Testament re-states some of the Mosaic commandments and blessings, and, by offering them to Christians to claim for their own, transfers them over into the Christian dispensation. Ephesians 6:1-3 re-states the promise of general blessing and long life for those who honor their parents. The New Testament cites certain other promises from the Old Testament, such as Peter positively quoting Psalm 33:12-16 as an object of Christian faith, in 1 Peter 3:10-12.
But the New Testament is selective in the way it applies the Law. The whole Old Testament is still part of our Christian training in righteousness (2 Timothy 3:16-17), but the NT is equally clear that not all of it is to be obeyed anymore. The apostle Paul and the other apostles tell us which of the commandments continue, and which of the promises apply to Christians. I don't know anywhere in the New Testament where the apostles say God's promise to give abundant children still applies at all, or, if it does, that it applies to Christians.
Lastly, the idea that Christians should try to re-conquer the United States by having lots and lots of children is unfounded. By God's design, Israel was a faith and a bloodline. The Christian Church is only a faith, and not a bloodline. The dividing-wall of bloodline, established by the Law, was abolished by Christ's blood (Ephesians 2:11-16). To the degree that we are like Israel, we are like Israel in the Diaspora (1 Peter 1:1 ) -- we're a nation without an homeland. The United States is not "ours".
Christians should absolutely reject the secular contempt for children and the family, and purge it out of our mental systems. It is a moral imperative that we think and feel about children they way God does. It is a moral imperative that all Christians be pro-life, pro-marriage, and pro-sexual morality, which includes a categorical rejection of homosexuality. But the New Testament doesn't command Christian couples to have "X" number of children.