Joe at Evangelical Outpost looks as if he's finally come around to what I and others have been arguing.
I doubt that gays and lesbians could do as much damage to the institution as we heterosexuals have done by allowing "no-fault" divorce.
He thinks a change on this issue will create a legal fiction (though I think it already is that legal fiction, and the legal fiction that it already is basically shouldn't give much reason to oppose at least civil unions). I may not even agree with half of what he's said on this issue, but I think this post raises some issues worth thinking about.
Meanwhile, Geoff Pullum at Language Log points out a fact about language usage that pretty much demonstrates that one way of framing this debate is linguistically naive. You can't just redefine a term with a law or an amendment. A law can't resist a change in language that dictionaries have already documented. Language doesn't work that way, and the French are in the process of learning that doing such things basically ensures that your language will die out. People who think they're debating over what the term 'marriage' in fact means should realize that it does in fact have multiple usages, as any good dictionary will reveal. You can talk about a marriage of minds, which is an extended definition originating as a metaphor. There's the legal usage, which includes whatever civil marriage allows, which varies from location to location. There's the traditional sense of the term, which doesn't allow same-sex marriage, but my use of the term in this very sentence shows that it also has a meaning that extends to same-sex unions as much as for heterosexual unions.
So Christians who want to oppose this can't see it as that. The issue is not over what the term in fact means. It's over whether the civil use of the term in legal documents should reflect the traditional concept of marriage or the more general concept that appears most frequently nowadays.
Now Pullum goes a little further with this than I would. He makes it entirely a civil rights issue, but whether it counts as that assumes one way of describing it, and what seems to me to be an equally good way of describing it allows no such classification. Here's how Pullum describes it:
It's about a denial of rights. The idea is that if you fall in love with a lesbian and want to marry her and live with her forever and share your life and property with her and be with her until you sit by her side at the hospital when she dies, that's O.K., but your rights will be subject to a limitation: you will be permitted all this under the sanction of the institution of marriage if you are male, but denied such permission if you are female. To add an insistence on that point in the constitution would be an act of discrimination, not of definition, so let's call things the way they are.
Here's the alternate way to describe it. The right everyone has in common is to marry someone of the opposite sex. I have that right. A gay man has that right. A lesbian has that right. A heterosexual woman has that right. Now not everyone will succeed at doing this, and not everyone will want to do this. That doesn't mean that people who want to do something other than that should have such a right. (This same issue came up for the gay sodomy case in Texas. See my response to that for more detailed reasoning paralleling this.)
Why should I prefer one to the other? More conservative people on this issue prefer the second way of describing it, and more liberal people prefer the first. According to the first description, preventing someone from marrying whoever they want is discrimination if there are others who aren't prevented from marrying whoever they want, regardless of any difference between the two cases. According to the second description, the difference between the cases is important enough that it doesn't count as discrimination. On any similar case, everyone has the same rights. I'm prevented from marrying a man, just as gay men are. Is that discrimination against me? Of course not, and it's not because I'm not gay and therefore don't want to do it. It's not discrimination against me simply because all relevant people have the same right -- the right to marry someone of the opposite sex -- and lack the same right -- the right to marry someone of the same sex.
Consider the following analogy. I have the right to take money from my bank account. I don't have the right to take money from anyone else's bank account. Now Hubert has the right to take money from his bank account but not from mine. What if Hubert belongs to a group, roughly 10% of the population, that wants to take money only from other people's bank accounts but not their own? We wouldn't allow it. Simply having the desire to do something is not enough reason to think there's a right to do that action. Both actions are taking money from a bank account, but there's an important difference.
Conservatives on this issue are saying that there is such a difference between same-sex marriage and opposite-sex marriage. It's not that one involves something directed at someone else who doesn't want it to happen, as with the bank account case. It does involve something they believe is wrong, and they do think it involves harm, though I would say that it's not a kind of harm that's as easily demonstrable on secular premises. The question that then gets raised is whether one group, with primarily religious motives, can determine that in a government that isn't allowed to set up a state religion. If it's a majority, why not? Determining the moral principles that will guide our country by having religious motives is exactly what the founders did. They believed stealing is wrong for religious reasons. They were in fact legislating morality based on religion. There's no constitutional or moral reason to disallow that. It's less appropriate in a pluralistic context where people will strongly disagree on values, but in a system that's largely democratic that won't happen as easily. That doesn't mean that it's in principle wrong.
So I'm still not saying this should be done. It certainly shouldn't be done as an attempt to say something about the language. If it's done it should be to retain a traditional concept of marriage as the primary one we use for legal purposes. I haven't thought Christians have any strong reason to pursue this over any other issues, but I do think there's nothing in principle wrong with the idea. (I certainly think mayors who forge state documents should be brought up on charges and pursued to the hilt, but that's no reason to think the people on the other side are any better in their hostility to allowing gay people to have legal benefits like providing health insurance to partners who are as committed as husbands and wives are and have children to support.) Whatever you think of what should be done, I think it's going to be very hard to show that the laws as they have been are discriminatory unless your description of the act presupposes something that the other side isn't willing to grant, and it's not something I'd even be willing to grant.