In knocking down the last remaining barriers to same-sex marriage in the United States, the Supreme Court announced a conclusion that, according to every measure of public opinion, a substantial majority of Americans have already reached: there is no longer any justification for denying the dignity and protections of civil marriage to gays and lesbians. The decision is consistent with the American constitutional principle that government must provide strong justification – that is, it must point to serious and concrete harms – before it denies a fundamental liberty such as the right to marry. On this point, the arguments of...