Experience suggests that most legal philosophers and ethicists are not surprised to be told that liberal states cannot permissibly prohibit same-sex marriage (henceforth: SSM). It is somewhat less clear what the appropriate liberal strategy is, and should be, in defense of this thesis. Rather than defend SSM directly, I proceed indirectly by arguing that SSM prohibitions are indefensible on liberal grounds. First, I articulate a principle that has roots in constitutional law that I dub the “Rational Basis Principle,” a principle intended to capture a constitutive commitment of liberalism: a commitment to liberty. The Rational Basis Principle condemns liberty-limiting legislation as indefensible unless that legislation bears a reasonably conceivable rational relationship to a legitimate state interest. I then argue that while SSM prohibitions limit liberty, they bear no reasonably conceivable rational relationship to anything that a liberal would regard as a legitimate state interest. Accordingly, same-sex marriage prohibitions are rightly dismissed as illiberal.