Got this from Jordan Ballor last week:
There are at least two ways I see that are helpful for getting at the question of the church's attitude toward the recognition of gay marriage by the government. I'll outline the short way, and give some hints at the long way, which I believe both end up arriving at the same conclusion.
Before this, however, I want to take a moment to define some terms. For the purposes of this discussion, I will use the term gay marriage to refer to the legal and public recognition of the union of two same-sex partners. My use of this term should not be construed as an acknowledgment of this social relation as a species of the genus marriage. I explicitly do not believe that so-called gay marriage is really marriage at all.
First, the short way. If we take Aquinas' rule of thumb as a starting point, that not all immoral things are to be illegal (quoted here), the determining factor becomes whether the criminalization of an immoral behavior would result in more or less evil, i.e. whether by doing so the state would be stirring up more evil or restraining it.
From a Christian perspective, that gay marriage is immoral is beyond doubt and that it violates at least the commandment regarding adultery is undisputed. Note that the scope of the seventh commandment is sexual purity, and that as has been the traditional Protestant and Roman Catholic practice is to interpret these commandments with both positive and negative aspects.
As part of the second table, Calvin, Luther, and others would agree that the enforcement of the adultery commandment at least theoretically falls under the purview of the state. The traditional differences which you relate that are often observed about the relationships between the first and second table are thus of no real relevance for this discussion, since first table commandments are not at issue. Gay marriage is not simply a "religious" issue as the first table is often construed, but a moral/civic one relating to the second table. With this in mind, we must at least consider the possibility whether or not homosexual activity, and certainly the kind of homosexual relations characteristic of gay marriage, ought to be criminalized.
Let's assume for the sake of this argument, as so much of American society already has, that this sort of legal prohibition does not meet Aquinas' prudential criterion: it creates more evil (in the form of an intrusive government, among other things) than it restrains. This, I believe, is a possible and tenable argument against the criminalization of homosexual activity.
This recognition does not leave us with only one option, that is, for the government to recognize gay marriage. This merely leaves us without any laws whatsoever from the government on this point, and thus leaves the government's approval or disapproval of this activity as moral or immoral ambiguous at best.
But for the government to actively recognize and therefore promote gay marriage would be to explicitly sanction this activity as morally praiseworthy, just, and helpful for society. We have already established that homosexual activity is immoral, and therefore the government has no valid role in promoting or establishing such activity as normative. There is thus a difference between saying something is legally permissible and that it is morally permissible or even praiseworthy.
In this way, the Christian view of the government's role regarding gay marriage can take two forms. First, the Christian might say that the government should prohibit and enforce this portion of the second table in pursuit of restraining evil. Second, the Christian might make a prudential judgment and say that the government would create more evil by making and enforcing such laws, and should therefore make no positive law on this point. There is no third option for the Christian view of the state, that is, that it should actively promote, recognize, and protect an immoral set of social relations.
I understand the current attempts of Christians to argue in favor of some sort of marriage amendment to prevent the third (non)option, in favor of the second, which leaves homosexual relations legal but does not allow for them to be codified and sanctioned by the state.
The long way of going about this argument would be to outline the roles and relationships between the various institutions, spheres, or mandates, specifically regarding marriage/family, government, and church. I understand these in a similar way to what Bonhoeffer says regarding the four divine mandates (marriage, work, government, church), which I believe is consistent with a line of social thought including at least one possible form of Kuyper's view of sphere sovereignty (not specifically Dooyeweerdian conceptions, for example) and going back through various Reformers including Wolfgang Musculus, who wrote of three laws in the Garden of Eden, relating to procreation, dominion, and work/provision of food.
On this account, marriage and family exist apart from and distinct from government and the church, and so both of these latter institutions merely recognize, affirm, and ratify the prior relationship rather than creating it de novo. I believe gay marriage would be a legal creation, not the recognition of an actual prior social relation that is continuous with the created and preserved order of heterosexual marriage. I don't have time more than to hint at this latter method, but again, the result would be that the state certainly has no obligation, or even permission, to recognize, promote, and/or establish, a set of social relations that violate the moral order, especially as articulated in the second table commandments.
A few other issues:
1. Can you explicate further this distinction between an instrumental vs. sacred function of the state to which you refer? Do you get this terminology from Luther? For Luther, the state is sacred, insofar as it is God's rule with his left hand.
2. The recognition of the state as an order arising from the Fall rather than something embedded in creation is of ambiguous relevance. Calvin would certainly agree that the coercive nature of government arises as a result of sin, even though he might argue that government without this characteristic is at least hypothetically manifest in the primal state. But Calvin is not representative of the entire Reformed tradition. A more modern example, like Brunner, who at least in some ways is taken to represent a Reformed position, argues explicitly that the state is an order of preservation arising after the Fall, distinguishing it from an order of creation.