The combination of only a man and a woman in a lifelong union is the only way to create more human beings in the most optimal environment for the most important thing for everyone in that family: keeping the faith. A commitment to one’s family nurtures one’s faith.
As Mary Eberstadt has documented, family disintegration and the failure of family formation are strongly linked to apostasy. If that is the case, then Christians need to be doing things like countering the cultural insistence that people wait until they are financially comfortable before starting a family and stay artificially infertile indefinitely to help that happen; making theologically robust Christian K-12 schools the top priority of evangelization efforts; and making it more institutionally possible for young people to get started in life without college loans.
It’s not clear how much American Christianity’s decline stems from unthinkingly accepting our culture’s antagonism to sexual fertility and our refusal to prioritize evangelism in the home, but it’s clear there’s a relationship between these that bears deep introspection.