In an interview last year, Focus on the Family head Jim Daly seemed to concede that same-sex marriage would be legal sooner or later. As I wrote earlier this week, that’s because younger Americans largely support the idea. In his interview, Daly suggested that Christians should focus on what marriage meant within their churches. “The piece of paper that you get at the state to recognize your marriage is worthless,” he said. “It’s like registering your car.”

Daly and I agree on that. Not that a marriage license is worthless from a legal perspective, by any means, but rather that it’s worthless from a spiritual perspective. Whether two people are in a state of holy matrimony is a purely private, religious matter. A government-issued license can do nothing to confer sanctity on a relationship. If a church does not consider a marriage valid in the eyes of God, that’s its business. I don’t have to belong to that church. There’s no sense fighting over whether allowing same-sex couples to legally marry diminishes the sanctity of marriage. Legal statuses aren’t sacred.

But as long as the government is going to confer on willing couples the legal status of marriage—and it certainly makes some sense to treat formally committed couples differently in the law—then all couples should have the right to marry. Requiring same-sex couples to call their relationships “civil unions” does little more than symbolically discriminate against them by creating a special, “separate but equal” status for gay people. Consider how offensive it would be to pass a law saying that African-American couples weren’t allowed to marry, but could only enter into civil unions. It wouldn't be the same.

As I have argued, the case against allowing same-sex couples the same right to marry as heterosexual couples is weak. In a recent essay on Big Think, my colleague Peter Lawler put forward the two strongest arguments in favor of restricting marriage to heterosexual couples. The first is that a heterosexual union is simply what “marriage” has historically meant. The second is that the authors of our Constitution never intended to grant a right to same-sex marriage. Both points are more or less true. But neither really weakens the case for allowing everyone the same legal rights regardless of sexual orientation or gender identity.

Peter is right to say that for much of history marriage has generally had something to do with heterosexual couples conceiving and raising children. But what does it matter if ancient Greeks or colonial Americans didn’t practice same-sex marriage? Like most human societies, both also allowed slavery and treated women as the political inferiors of men. These were not high-water marks of human morality. I don’t need tradition to tell me that slavery and gender discrimination are wrong.