Someone asked me recently how to answer a question about what the Old Testament teaches about marriage. An advocate of same-sex marriage claimed that the Bible doesn't teach marriage is one man and one woman because the Old Testament has polygamy. Leaving aside the fact that even in polygamy (multiple marriages), each marriage is still one man and one woman, there's a fundamental principle of Biblical interpretation that we have to keep in mind on this topic and many others.
The Old Testament is largely historical narrative; it's simply reporting what happened. The mistake many people make about the Old Testament is taking everything to be moral commands when it's often reporting history. Never in any of the accounts about the cultural marriage practices of the time are they approved of or communicated in a way that makes them examples. In fact, often the trouble that comes from the polygamous marriages can be taken as examples of what happens when we engage in cultural practices that break God's law.
About the only place in the Old Testament where we have any textual reason to derive a moral imperative about marriage is in the creation account because it's the way God created it and said it was good. In the New Testament, Jesus refers back to this, saying it was so from the beginning - a man and woman. That's the design that has moral imperative.
So people who try to use historical accounts as moral guidelines are just misunderstanding or abusing the text.