From the Des Moines Register:
As academic biblical scholars, we wish to clarify that the biblical texts do not support the frequent claim that marriage between one man and one woman is the only type of marriage deemed acceptable by the Bible’s authors.
The fact that marriage is not defined as only that between one man and one woman is reflected in the entry on “marriage” in the authoritative Eerdmans Dictionary of the Bible (2000): “Marriage is one expression of kinship family patterns in which typically a man and at least one woman cohabitate publicly and permanently as a basic social unit” (p. 861).
The phrase “at least one woman” recognizes that polygamy was not only allowed, but some polygamous biblical figures (e.g., Abraham, Jacob) were highly blessed. In 2 Samuel 12:8, the author says that it was God who gave David multiple wives: “I gave you your master’s house, and your master’s wives into your bosom. ... And if all this had been too little, I would have given you even more” (Revised Standard Version).~Ezra 10:2-11 forbids interracial marriage and orders those people of God who already had foreign wives to divorce them immediately.
So, while it is not accurate to state that biblical texts would allow marriages between people of the same sex, it is equally incorrect to declare that a “one-man-and-one-woman” marriage is the only allowable type of marriage deemed legitimate in biblical texts.
This is not only our modern, academic opinion. This view of the multiple definitions of “biblical” marriage has been acknowledged by some of the most prominent names in Christianity.
That's the problem with using the Bible as a justification for certain things. Sometimes, the Bible can be a Pandora's box, and the consequences of basing laws on those things can be worse than you intended.