Christianity sees the family as society's foundational sphere---the primary community for bearing and raising children, through which culture is built. Libertarianism, lacking a full worldview, views the family mainly through rights (parental self-ownership over children) and economics.
It is in raising families that needs for sufficiency and efficiency are made most plain. We find that we need innovative answers to storing food, building shelter, finding suitable transportation, and so on. The raising of families is what creates needs within the market for innovators and entrepreneurs to respond to. Often, parents themselves become the entrepreneurs who solve the problem and then offer it for sale. And this pattern continues as families come across new problems, new solutions, new innovations, and new goods and services.
Those who are unmarried benefit from having the economic needs of families met. Take the production of food, for example. Roughly a century ago, many families---particularly in rural areas---produced much of their own food. That task was far easier when the work was spread among the family, rather than by a single individual living alone. When people started producing more than they themselves needed and traded and sold their surpluses, that benefited those without families.
The state repeatedly invades this sphere because the family is so vital. Libertarian Christians affirm not only its practical and economic importance but its spiritual, relational, emotional, and psychological primacy. Many libertarians do not. This gives libertarian Christians a unique opportunity to persuade the broader libertarian movement that the family---prior to the state---is the true foundation of a free and flourishing society.
This question is from Faith Seeking Freedom: Updated & Expanded, available in paperback, PDF, and Kindle.