Think about it, we are already asking the question meaning it is very possible. I don't like the idea of polygamy because that could easily lead to other messy issues like forced marriage or violent cults as it has in the past. I think polygamy should remain illegal in the United States. However, I have no issue with incestuous marriage as long as they do not reproduce because that leads to crippling genetic mutations to the child, but they can always adopt.
So, I would like to ask another question: Would it be bad for these other forms of marriage to be accepted into our society; or at least bring up the serious discussion? I think it is good to at least consider it and let it become more accepted with the future generations. I think it is wrong for us to force the overly conservative views of our own dying generation on future generations because that only inhibits the evolution of society.
I would like to point out that marrying your cousin is already legal in twenty states without restriction, bestiality is legal in eighteen states/territories, and while polygamy is not legal in any state people still continue to practice it, albeit in secret. It's outrageous to assume homosexual marriage would lead to things that already take place, especially when a lot of the states that ban homosexual marriage such as Alaska, Alabama, Colorado, Florida, and Kentucky already allow you to either marry your cousin or have sex with an animal and as I already stated, more likely than not somewhere in those states people practice polygamy.