No, really?
Polygamists Celebrate Supreme Court’s Marriage Rulings
I've said it before, I'll say it again: In logic, slippery slope is a fallacy. In politics, it is a certainty.
And polygamy is toxic to any culture that tries it.
Western civilization is the historical and geographical exception to the rule, and it is founded on creating a broad middle class who will work hard and produce. That does not happen when you have an all-or-nothing system of mating arrangements.