The New Testament, and therefore the New Covenant has a lot of different things to say about marriage without being especially precise. including Jesus' very clear statement in Luke 18:29-30 that any man who leaves his wife and children behind for the sake of the Church shall be rewarded all the more in heaven. Sounds a lot like abandonment or divorce to me. The traditional one man, one woman form is quite clearly assumed or explicated in the various texts, but not inviolably so.
But since Christianist opposition to gay marriage hinges substantially on Leviticus 18:22, which is in the Old Testament, it seems to me that any effort to understand the Biblical definition of marriage should rest on the same foundations. This is simple fairness and intellectual consistency, after all. (With respect to the New Testament, Romans 1 26:28 is often cited, but if you read the whole passage and apply a little bit of context, it's a larger discussion of idolatry and turning away from God and a fairly long list of things which are disapproved of, including pride, boasting and backbiting. It's certainly not the explicit legalistic prohibition against homosexuality found in Leviticus.)
And Biblical marriage in the Old Testament is a messy, complicated thing.
In Genesis 11 through 25, Abraham rocked it with Sarah and Hagar. Definitely not one man, one woman. For bonus points, Sarah was his half-sister. Admittedly, he wasn't formally married to Hagar, but this three-way relationship was pretty clearly part of God's plan.
In Genesis 25 through 50, we learn about Jacob. He rocked it a lot harder with his cousins Rachel and Leah, and various servant girls, all of whom the Bible clearly states were given over to him in marriage.
In the story of David recounted in 1 Samuel and 1 Kings, the foreskins of the Philistines are named as a bride price for his wife Mical. Later on, David arranges the death of one of his generals so he can marry Bathsheba, the man's wife. Neither of these seems to an approved modern method of courtship. He ultimately winds up with eight wives.
In 1 Kings, Solomon is described as having seven hundred wives and three hundred concubines. Definitely not one man, one woman.
In the book of Ruth, Ruth's relationship with Naomi is described using the same Hebrew words that describe Adam's relationship with Eve. Even through millennia of selective editing, this seems highly suggestive of a same sex relationship.
This doesn't even get into the issues around Lot's daughters, for example.
All of which to say, Biblical marriage is not clear cut. Since my Christianist friends place so much weight on the Old Testament condemnation of same sex relationships in defending traditional marriage, I think it's only fair that the place a similar weight on the Old Testament's highly colorful and varied definitions of marriage. One man, one woman isn't a simple ideal, and it certainly isn't God's law.
Doubtless there are detailed theological arguments that richly justify how one picks and chooses which Old Testament verses to defend to the death as inviolable holy writ, and which to blithely ignore. I'm just as certain that once you take even one step away from the moral absolutism of Biblical inerrancy, for example, by wearing mixed fabrics, you lose the right to call upon individual "clobber verses" as being the final arbiters of God's will with respect to whatever particular argument you wish to make.
Me, even as an atheist I'm a lot more in favor of the New Testament's messages of love and fairness and non-judgmental inclusion than I am in favor of carefully selected Old Testament prescriptivism. I'm pretty sure that's the whole point of the New Covenant. Which would seem to argue for a much broader Biblical definition of marriage than my Christianist friends insist upon. Or at the very least, a much kinder and more tolerant treatment of people of whom they do not approve.