You are correct, polygamy is the next step.
There are groups in Canada (Yes, I know they are not part of the US ;-)), that are suing the government with that very argument, since homosexual “marriage” is acceptable, you must permit us the same “rights” by allowing polygamy.
I think the push for polygamy will come from the Muslims. They are a favorite victim group of the Left. And after that, they will push to lower the age of consent for marriage.