Katherine Shaw Spaht, in her Meaning of Marriage essay on how law has been abandoning marriage, cites an important step down the slippery slope in one of Justice Thurgood Marshall's opinions. In the 1978 case Zablocki v. Redhail, Justice Marshall wrote, "it would make little sense to recognize a right of privacy with respect to other matters of family life and not with respect to the decision to enter the relationship that is the foundation of the family in our society" – the decision to marry. I normally agree with Justice Marshall, but in this instance I think he is exactly wrong.
Marriage transforms individuals and society because it is a public vow. We recognize marriage as a status apart from all others, as making two persons into one flesh, because it is a public promise by the couple and by society. That vow carries with it many grave responsibilities, most importantly to raise any children that the marriage might produce. Raising children well is a grave public duty.
We accept the privacy of what goes on in families more than in any other institution. Family privacy is not absolute; other institutions will intervene if family members are beating one another. But we grant the most leeway and place the most trust in families to do in private what they want because we believe that, ultimately and in the main, that private freedom will most enhance the public good.
Society has a great interest in marriage as a public act and public status because marriage is a shield behind which people have privacy. If the state treats marriage as just another private choice that individuals might make, society loses the public benefit of marriage, and marriages lose the trust that allows society to respect their privacy.