Courts already outlawed polygamy so we are talking about same sex marriage here between 2 people.
Or rather, the courts upheld states outlawing polygamy- acknowledging the power of the states to make decisions in such matters.
And in regard to your argument about a constitutional right to gay marriage I have acknowledged several times over that you are in a good place on that because the court is with you on that - unless and until it is not- just as with Roe.