With the right to gay marriage in the bank the next frontier is polygamy. It only makes sense. Recognized major world religions have forms of polygamy. A reasonable argument for having more than one wife can be found in Islam and Mormonism.
But the larger argument is that if marriage is not just one man and one woman, joining in a partnership for the purpose of forming a family and child rearing, then who decides what it is? The likely answer is that the participants do.
Groups of various sizes and genders/proclivities are going to challenge the court system and will win. Why not? They say they want to be left alone to live their lives as they see fit.
This is exactly what I want, also. Don't infringe my rights, especially the clearly enumerated rights like the right to free speech, the right to bear arms, the right to be secure in my person and my papers, and the others that the Founders enshrined in clear words, and I won't infringe yours. It's like a social contract.
And good luck sorting out communal property when y'all decide to get divorced. It's messy enough with two people.
The smallest minority on earth is the individual. Those who deny individual rights cannot claim to be defenders of minorities.