Throughout human history, all societies restricted marriage on the basis of age, gender and number. Those instances where restrictions based on class, race, religion or other factors were imposed, those did not last long before those societies removed them, but age, gender and number are nearly universal and permanent.
Such restrictions on marriage are within the authority of all societies to impose on themselves. If American society as a whole had decided to remove restrictions based on gender, they had the authority to do that. It would seem apparent that there was some movement in that direction during the last decade, and had it been allowed to develop by persuasion, probably would have resulted in same-sex marriage. And it would have thus upheld the authority of society to make such judgements, preserving the legitimacy of restrictions on age and number.
But once discrimination law was invoked to invalidate the very foundation of restrictions on adults to marry, a whole different legal precedent was established. If denying marriage to adults amounts in and of itself to illegal discrimination, then that applies equally to reasons of number as to reasons of gender. Before the law, they are identically discriminatory.
One cannot argue that social mores support restriction on number, because the same-sex movement denied the legitimacy of social mores as a factor. One cannot argue that it is harmful to our society to allow multiple people of whatever genders to marry, because impact on society was denied by the same-sex movement as a legitimate factor. One cannot argue that a family unit where multiple people can enter or leave at will (after passing through divorce court of course), creates a legal and human mess of the concept of family as a stable child-rearing institution, because the same-sex movement eliminated the question of child welfare as a factor.
All of those questions are important to a society’s judgement on marriage, but the process chosen by the same-sex movement, court rulings based on discrimination law, invalidate all those questions as germaine and relevant. Those questions could not even be asked, let alone discussed. Have you forgotten how it was? Now we are left with no consistent response to the charge that restrictions of marriage based on number are inherently discriminatory. We are left with no recourse but to say “society does not want plural marriage”, but of course, that too was discredited by the way same-sex marriage was pushed through. What society wants is not longer important.