It makes no more sense to consult the Bible on the question of how to define civil marriage than it does to consult the Bible on the rules of 21st Century maritime trade in the Mediterranean Basin. The kinds of relations and ideas that constituted that culture is dramatically different from every culture of today.
If the church is counter-cultural, why should it invest so much time and energy on the civil institution that turned a covenant into a contract, overseen by the state, and which has existed in its current form for a short while.
The first marriage license was not issued until 1639 in Massachusetts. Marriage as a love-match - as opposed to family arrangements and other considerations - didn't catch on until the mid-18th Century, and then only among the well-to-do.
We didn't nuclearize the family until after we A-bombed the Japanese.
Marriage is not natural, and never has been. It is absolutely cultural, even when that culture is "individualist."
There are people who would deny that here are men and women, an extrapolation from the cultural construction of marriage, and I disagree with them, too.
Culture and nature apart is just another dualism.
We are material beings; we are a particular kind of creature, and one that reproduces ambisexually. Having the requisite physical characteristics, most individuals qualify as reproductively female or reproductively male when they reach sexual maturity. That does not in any way diminish the essential humanity (in the eyes of God) of anyone anywhere who falls outside these two sexual majorities. We would not be having this conversation, in fact we would not be at all, if it were not for reproductive females and reproductive males.
It does not follow, however, as some would have it, that marriage is natural in some way prior to culture. Human beings are naturally cultural. We are biologically determined to not be biologically determined.
When Jesus spoke about marriage, he was not speaking to a 21st Century white couple in the American suburbs, with two kids, a mini-van, and a Labrador Retriever. There was not the least similarity, apart from a male-female dipole. Jesus spoke to men who sometimes had more than one wife. The wife was considered the property of the man, and without his patronage - in the case of divorce, for example - a woman who was divorced (a short and summary proceeding initiated by the husband) was at risk of becoming a beggar or a prostitute, and her children could be put out with her.
You have privilege, Jesus told men. Don't abuse it by abusing your wife, by subjecting her to injustices. Anyone who thinks Jesus had no sense of humor, remember that he said, "Gentlemen, you may not throw your wives out in the street just because you're feeling ornery. She better be running around on you before you resort to divorce." So far, so good. Jesus was a compassionate man. But then, he says, "Just to be fair, though, your wives can divorce you if you run around, too."
What the hell!!! would have been the response, because he spoke as if women had any agency at all. It was a scandalous thing to say.
There husband had power over his wives, and as Jesus did with a number of norms of his day, he revealed it as a species of domination.
We've not been habituated to the intuition of a 1st Century Palestinian Jew. We do not intuit that this is a shockingly inappropriate thing to say to that audience of Judean men two millennia past.
Instead, we fashion an anachronistic rule out of this story, a rule regarding "marriage" and "divorce," then continue to apply the same rule to these to names, even as they now name completely different phenomena.
In the "Marriage" class I attended as part of my Rite of Initiation, we were told not to be homophobic (good advice in my view), but that for us, marriage is unitive (as in "till death do we part") and generative (it makes babies). That is the basis of the Sacrament of Marriage (which need not, in my view, be sanctioned by or incorporated into the secular liberal state).
If there were no state, surely we could still have the Sacraments, by our own lights. And marriage need not be for everyone (celibacy and-or singleness are options we ought to take a good deal more seriously).
For this reason, I don't think our responsibility is to fashion the state's definition of marriage. We don't need the state, and we don't want the state telling us what is and what is not sacred. We do, however, have a responsibility in my opinion, according to the example of Jesus, to reveal injustices.
The question of gay marriage ought not be Christians telling other people what the secular definition of marriage ought to be - we have our own. We ought to look at law as it exists and seek to know and reveal its injustices in the same way that Jesus witnessed against hard-heartedness.
I believe non-heterosexual people have suffered and still suffer injustices; and I believe marriage law, such as it is and recently has been, has been a source of some of those injustices. I do not consider it my responsibility as a Christian to dictate to the wider society.
[I can question my church's categories, unitive and generative as being inherently about a man and a woman, with examples like a heterosexual couple who is infertile, for example, and how that differs from a non-manipulative relationship between two homosexual partners who adopt children. But that's another debate.]
The relationship of adults and children to the state is of interest to me, as a follower of Jesus, because the state is discriminating against some people in matters that have real, material, and harmful consequences for them. If we are talking about a Christian Sacrament (marriage), why would we try to tell non-Christians how to participate in it?
That doesn't seem complicated to me. As a Christian, I don't take my mission to be managing the law, engineering society, or taking political power. But when a law is harmful, as I believe DOMA was, then I have to say, it's a bad law, and I'm glad it's gone. (DOMA is what gave rise to this post, DOMA and the conflicts it raises for my fellow Catholics.)
The idea that gay marriage might "destroy" marriage seems pretty silly, too, when the supposedly sacred institution is a legal document issued by a County Clerk, that nowadays is the basis for future litigation. Civil marriage is desacralized already.
But there's nothing "natural" about it. Nor is there anything natural about the Christian marriage as sacrament, given that sacrament as instrumentum came about only in the 13th Century, so the follow-on discussion, once we untangle this constantinian mess between church and state (where churches want to have their cake and eat it too), perhaps we can talk about our beliefs and practices as Christians with regard to gay Christians, who - as one friend put it - "find it weird to be somebody that can only get married /outside/ the church!!"