We’re having a robust national debate over whether marriage should be redefined to include same-sex relationships. It’s an important debate. And in many ways — despite what some activists say — it’s only beginning.

Whatever the outcome of the Supreme Court’s deliberations, the only thing that’s inevitable is this: Americans will keep talking about marriage well into the future — and with good reason.

The nine justices are considering challenges to state and federal laws defining marriage as the union of one man and one woman. The two cases before it provide an important opportunity for Americans to reflect on three questions: What is marriage? Why does it matter for public policy? And what are the consequences of redefining marriage?

Appeals to “marriage equality” make for good sloganeering, but sloppy reasoning. Every law makes distinctions; equality before the law protects citizens from arbitrary ones. Marriage equality demands knowing what marriage is.

Marriage exists to bring a man and a woman together as husband and wife to be father and mother to any children their union produces. Marriage is based on the biological fact that reproduction depends on a man and a woman, and on the social reality that children need a mother and a father.

Marriage predates government. It is the fundamental building block of human civilization. All Americans — not just conservatives — should respect this crucial institution of civil society. Indeed, 41 states affirm that marriage is the union of a man and a woman.

Government recognizes marriage because it benefits society in a way that no other relationship does. Marriage is society’s least restrictive means to ensure the well-being of children. State recognition of marriage protects children by encouraging men and women to commit to each other and take responsibility for their children.

While respecting everyone’s liberty — after all, nothing is made illegal by marriage laws — government rightly recognizes, protects and promotes marriage as the ideal institution for childbearing and childrearing.

But, redefining marriage to include same-sex relationships would further distance marriage from the needs of children. It would deny — as a matter of policy — the ideal that a child needs a mom and a dad.

Decades of social science show that children tend to do best when raised by a married mother and father. The confusion resulting from further delinking childbearing from marriage would force the state to intervene more often in family life, prompting welfare programs to grow even more.

In recent years, marriage has been weakened by a revisionist view that is more about adults’ desires than children’s needs. Americans increasingly are tempted to think that marriage is simply whatever sort of relationship consenting adults — be they two or ten in number — want it to be: sexual or platonic, sexually exclusive or “open,” temporary or permanent.

Redefining marriage would put a new principle into law — that marriage is whatever emotional bond the government says it is. No principled reason could be offered for why an emotional union should be permanent. Or limited to two persons. Or exclusive.

But marriage can’t do the work that society needs it to do for generations to come if the norms are weakened further. All of us who care about a thriving civil society — with institutions capable of limiting the state and its power — should be alarmed.

All Americans have the freedom to live as they choose. No one, though, has the right to redefine marriage for all of us.

-Author Ryan T. Anderson is the William E. Simon Fellow at The Heritage Foundation. He is co-author with Sherif Girgis and Robert P. George of the book “What Is Marriage? Man and Woman: A Defense” (Encounter, 2012).