Of all of the Bible-based arguments against same-sex marriage (which the Bible never, at any point, specifically prohibits, though it clearly would frown on the partners therein having sex), the flimsiest is the claim that "The Bible says that marriage shall be between one man and one woman."
The problem with the argument is that the Bible does not, in fact, say that anywhere.
It's just one of the many Bible citations that are widely quoted but that are actually not there, like the ever-popular "God helps those who help themselves," which appears nowhere in the Bible. Various people through history have expressed that sentiment. The first to use that exact wording was English political theorist Algernon Sidney, and Ben Franklin used it later in Poor Richard's Almanack. But the Bible? No.
On the subject of husbands and wives, I want to start by focusing on the very first mention, which comes almost as early as possible, in the second chapter of Genesis -- Gen. 2:22-24, to be specific: "Then the Lord God made a woman out of the rib he had take out of the man, and he brought her to the man. The man said, 'This is now bone of my bones and flesh of my flesh; she shall be called "woman," for she was taken out of man.' For this reason a man will leave his father and mother and be united to his wife, and they shall become one flesh."
There is one real oddity here: The idea that Adam would talk about a man "leav[ing] his father and mother" is hard to account for, coming from a man with no parents and considering that there had never been, at that time in human history, any fathers or mothers. (Pointing out that in Adam's case God was the father is not helpful, since if that's what Adam meant he would then be saying he should leave God.)
For my own purposes here, I just want to point out that the verse does not associate the word "marriage" with husband and wife, and doesn't in any way hint that the man couldn't have some more wives.
A Biblist will respond to me that there are plenty more verses about marriage as the Bible continues on. There's a listing of verses related to marriage in the article "Marriage Bible Verses", on the website biblestudytools.com. This isn't a collection of all of the verses that mention husbands, wives, or marriage in any possible context, but is only the ones that give descriptions and rules relative to marriage.
Looking through the list, I notice the following:
None of the verses really clearly define what the words "husband" or "wife" mean, though one might conclude by implication that "wife" means "a woman who has married" and husband means "a man who has married": the Proverbs 12:4 quotation clearly uses a male pronoun for "husband" and female pronoun for "wife" (and it's not alone in doing so). And certainly, reading the verses, they do imply that a wife may be married to a man, and that a husband may be married to a woman, and indeed the writers clearly assumed that that is the case. But any statement that a man may only marry a woman, or that a woman may only marry a man, is absent.
Especially damaging to the "One man, one woman" claim is the absence, anywhere in the Bible, of any prohibition of a man who is married deciding to marry again, without any termination (by death, divorce, or any other means) of the previous marriage. Same-sex marriage opponents often make use of "slippery slope" arguments -- that is, claims that allowing same-sex marriage will lead to other things that any rational person would find horrifying. The most amusing, to me, is the one that says "If we allow same-sex marriage, thus making such a major change in the meaning of marriage, then how could we oppose multiple marriages? If a man comes in and wants to marry two women, how can we appeal to any tradition if we've thrown tradition aside to allow same-sex marriage?"
How indeed. The argument rests on the assumption that no one wants to see multiple marriages, and concludes that therefore we can't allow same-sex marriages either.
This overlooks one glaring Bible fact: that men having several wives is commonplace throughout the Bible, and that, far from being angered by it, God actually gives some rules for how it should work. An ideal example of that is in Exodus 21:10-11 -- "If [a man] marries another woman, he must not deprive the first one of her food, clothing, and marital rights. If he does not provide her with these three things, she is to go free, without any payment of money." Why, thank you, God, that's actually quite feminist of you. I'm sure the Bible patriarchs with multiple wives, such as Jacob, who fathered twelve sons by his wives, the twelve sons being the founders of the twelve tribes of Israel, adhered to God's rules for multiple marriage to the letter.
Looking back at that page about marriage from biblestudytools.com, there's something I can't help wondering. The page says, in its brief introduction, "Marriage is a sacred vow between a man and woman and the Bible offers many verses that offer guidance for married couples, husbands, wives, newlyweds and engagement. Read verses from the Holy Bible about marriage in relation to God, Jesus Christ, and the Christian faith."
I've already addressed the point they begin with, stating that "Marriage is a sacred vow between a man and a woman..." I've noted that the verses quoted fall short of supporting that interpretation, other than grammatical implications that the writers are referring to unions between a male and female, without specifically saying that no other unions are allowed. But what I am wondering about now is the selection process involved in deciding which verses have been included in this list. As I said above, the list contains verses with descriptions or rules about marriage, and there is no indication that the list is incomplete. (It even goes so far as to include Proverbs 30:18-19, which actually makes no mention of husbands, wives, or marriage at all, and the presence of this Bible quote here is puzzling.) But whether or not the list is intended to be complete, it obviously is not, since it somehow manages to leave out that Exodus verse I just quoted above. Why is that, people at biblestudytools.com? The content of Exodus 21:10-11 is exactly within the range of the subject matter of the verses you are compiling on your "Marriage Bible Verses" page: it clearly states one of God's commands about marriage. Is it that you feel uncomfortable with that verse, and didn't want it on your list?
A Bible supporter picking and choosing verses that support his point and ignoring verses that don't. Imagine that.
Yet somehow, perhaps through carelessness, a verse that hints at multiple marriage did manage to slip through the net and make it onto the page. Look at Proverbs 5:18-19 -- "May your fountain be blessed, and may you rejoice in the wife of your youth. A loving doe, a graceful deer -- may her breasts satisfy you always, may you ever be captivated by her love."
Aside from being surprisingly hot, there is something else interesting in the wording of that verse. "The wife of your youth"? The first of your wives, the one you married when you were young? Why does "wife" need a qualifier, "of your youth," if you're only supposed to have one?
No doubt the writer of that proverb had just had an exceptionally good morning, but I hope he had been diligently following God's command to give some attention to his other wives as well.