The same holds true for most every culture on earth. Out of 1170 societies recorded in Murdocks Ethnographic Atlas, polygyny (the practice of men having more than one wife) is prevalent in 850. Even our own culture, which has an astoundingly high divorce and remarriage rate, practices a form of serial polygamy.The illegality of bigamy has more than merely a religious foundation.
Laws against bigamy predate Christianity.
Our Constitution requires that our laws have a *secular* purpose, not merely a religious one. So it's irrelevant as to whether Christianity supports or opposes any particular law.
The U.S. Supreme Court has determined that the Establishment Clause of the Constitution requires this.
The reasons for favoring gay marriage while excluding polygamy are completely arbitrary and based on personal preference. If you truly believe that gays have a legal right to marry then you have no grounds for barring polyamorous groups from doing the same.
What's that 72% or so?