The argument that made gay marriage law in America was about marriage equality: not special rights for gay people but the right to the same treatment. There may be something to regret in the smallness of this ambition (previously), but the argument is sound.
Denying people the right to marry requires a public justification -one that would be in principle convincing to all concerned - and this demands more than the expression of the private moral beliefs of a politically dominant group like fundamentalist Christians. What this comes down to is the moral and political significance of the challenge Why not? for public reasoning in a liberal society, for identifying and overturning unjust laws and policies.
In the wake of the US Supreme Court's decision it is therefore rather disconcerting to find various gay marriage advocates, such as Jonathan Rauch, arguing so badly - basically sneering - against polygamy. One has the impression of a drawbridge being raised up. As far as I can tell, polygamy presents as good a case as gay marriage for legal recognition and regulation by the state, as Justice Roberts' Supreme Court dissent pointed out.