"Don't attempt to legislate your Christian morality," we're told. "This is a secular nation, and the Bible isn't the law of the land."
It's a valid point, at least at first glance. So how do we respond?
As Trevin Wax observes in this video discussion with Kevin DeYoung and Collin Hansen, legislation is always, inescapably moral. The real question, therefore, is not whether we should legislate morality but rather whose morality we should legislate. All laws are morally freighted and, in some manner, discriminatory. Moreover, the idea that traditional marriage is simply "a Christian morality being enforced ignores the fact this isn't just a Christian point of view, but one held by the vast majority of people in the world."
Further, DeYoung points out, the purpose of the law is not only to protect rights, but to teach. And it takes some audacity, he adds, to dismiss the traditional view as "bigoted" considering the fact that "until 15 years ago no country we know of had this alternate form of marriage." It's imperative we rightly answer two questions: (1) What is marriage? and (2) What is the government's role in marriage? To the latter, DeYoung observes that the state's interest lies in incentivizing whatever type of family structure is best for "societal stability and human flourishing."
But what's our standard for making such determinations? As Hansen notes, the question of final authority is a critical one. As Christians, how do we help people find an authority outside of themselves?
To read more on this question from a well-informed, non-Christian perspective, consider Michael J. Sandel's Justice: What's the Right Thing to Do?.