This argument assumes that Christians can maintain and safeguard their own definition of marriage by refusing to impose a particular viewpoint in the public square. Often with good intentions, some Christians wish to privatize marriage into a strictly ecclesial practice, treating it like we would the Lord's Supper or baptism.
But therein lays the problem: The church's theology on marriage, while certainly ecclesial, isn't sectarian. Marriage leads one outside the walls of the church and into the public square because marriage, by design, reveals a certain cosmology about our essence as being made male and female. Marriage has an innately public purpose by bringing together the two halves of humanity. If you embrace man as man and woman as woman, you might be on the losing end of a culture war over marriage, but you'll be on the side of truth when the dust settles about human nature.
One Type of Marriage
The government is not in the business of upholding theological positions or propagating sectarian ethics. The government forbids stealing, for example, not simply because the Decalogue forbids it, but because stealing violates the public trust. Because stealing undermines cooperation and a well-ordered civil society, common belief about the harms of theft leads to outlawing it. Of course, as evangelicals, we believe everything has God as its author, and so we view stealing as breaking God's commandment. But that is not government's interest in making theft illegal.
While marriage may be ultimately Christian, it's not exclusively Christian. Arguments that conflate theological meaning with direct public application ignore this division and treat a theology of marriage as akin to a theology of baptism. How a church administers baptism, however, is an ecclesial ordinance where the church marks out its members. The same cannot be said about marriage. It is entirely permissible for the government to uphold a view of marriage that comports with theological truth, but that is not held or promoted for theological reasons.
When we speak of marriage as only a theological construct, we do a disservice to the institution's public significance. There aren't two kinds of marriage—one secular, one sacred. There's only one marriage with one purpose, regardless of how different religious traditions handle or interpret the institution. Government does not uphold a particular theological interpretation of marriage; it upholds a view of marriage that differing theological and non-theological systems rightly accommodate. That's why civilizations across human history—some of them irreligious—have acknowledged marriage.
As Christians, we understand that marriage reflects the deepest truths of the gospel. As Christians in America, we also understand that government has an interest in promoting marriage as a social policy apart from any theological backdrop.
Marriage for the Common Good
If marriage policy actually matters for all persons, not just Christians, how should we understand the social purpose of marriage?
Government ensures and facilitates access to the institutions that benefit the common good. As a matter of social policy, we have to ask what marriage is and why marriage is important to the common good. Marriage is a natural and public institution whose purpose—at least in the interest of the state—is to make a man and a woman father and mother to any children their union produces. Government recognizes marriage because this function of the institution benefits the common good. Marriage, for instance, is the greatest weapon against childhood poverty.
While marriage physically unites a man and woman, it also aims to ensure child welfare and protection. We know from numerous studies, which use the best research methodologies, that children do best when raised in a home with a married mom and dad. There simply isn't a substitute.
None of these arguments is theological in nature or based upon Scripture. Instead, they appeal to our shared moral grammar as Americans. But none of these arguments contradicted Christian truth claims about marriage, either.
When the church declines to speak the truth about marriage, it invites competing and false views to rob marriage's purpose. Were the church to "get out of the marriage business" as some are tempted to demand, two mistakes will follow. First, the church will allow a false understanding of marriage to dominate the public square. Second, the church will becomes a secularized version of itself. Christians long ago insisted that a culture of no-fault divorce would not affect Christian marriages. But today, we're all too familiar with the testimonies of scarred Christians who have endured divorce. The reality of divorce within the church bears out this truth: If the church is not holding fast to the truth of marriage, it will bend and accommodate itself to the dominant marriage ideology of the public square.
In truth, if marriage loses, it's not just Christians who lose but all of society. Society will be deprived of the norms that lead to healthier cultures and sound social policy, of which marriage is the first and most fundamental.