Interesting article here by John Milbank of Radical Orthodoxy fame:
He makes the point regarding gay marriage that the redefinition of marriage by law ultimately redefines heterosexual relationships in terms of homosexual ones. He also notes that properly speaking, marriage doesn't fit all that well with gay relationships and that homosexuals themselves historically haven't really wanted to get married. Hence, there must be an ulterior motive. Ultimately, Milbank argues, the whole issue is about the divinity of the state. If the secular state can redefine an institution that exists prior to it (and was created by God, I might add), then it means that it possesses an absolute power over human life. Hence, Milbank see the issue has revolving around the deification of the state.
I think the question of the continuing deification of the state in modern life is a interesting one, and I've explored it here: http://jackkilcrease.blogspot.com/2012/07/thomas-hobbes-as-theologian-part-i.html and also here: http://jackkilcrease.blogspot.com/2012/07/law-gospel-and-liberal-tradition-of.html. Ultimately, in terms of winning the argument about traditional marriage (by which I mean both the belief in heterosexual marriage, and also the indissoluble nature of marriage- i.e., no divorces without Jesus' single exception) it is a daunting task for modern Christians. It is a daunting task because even before the debate begins, Christians are faced with the fact that everyone (including Christians themselves) already have a distorted understanding of marriage.
Prior to the modern era, the basic conception of marriage in Judeo-Christian culture was an Order of Creation and an economic relationship. Since all property was tied up in land, and land was owned by families, marriage was a way of ensuring intelligent and rational means of wealth transference and (depending on the status of the family) political alliances. Theologically speaking as well, love was secondary in the definition of marriage. In Luther's commentary on Genesis and in the Catechisms, he understands marriage as an Order of Creation established by God, and definitional of the human self in this age. Here he echoed Jesus in Matthew. Similarly, the RCC understood marriage as both an something rooted in creation, and elevated by the order of grace.
Though I may disagree with this later definition theologically, the commonality between it and the Lutheran one is clear: Marriage is a reality rooted in a legal, creational, and economic relationships. It isn't about the subjective feeling or personal preference of the participants. People in the pre-modern world of course did experience romance and love (it being universally human phenomenon), but such realities only had an incidental relationship to marriage. For perspective on this, read some of the Medieval chivalric romances: The authors actually assume that love and romance are only incidental to marriage, or in very extreme versions, very nearly impossible within marriage.
Things changed in the 19th century. Since Capitalism made wealth transference and generation possible without people handing it down through kinship, western European and American society had to come up with a new rationale for marriage. This rationale was companionship and romance, and marriage therefore was redefined as a public ratification of one's subjective romantic feelings. After this, divorce became more common. Why? Because if one doesn't have the experience of proper companionship with one's spouse the whole relationship isn't serving its function. Hence, why not just move on? Of course there were still legal barriers to divorce, but after the 1960s and no-fault divorce kicked in, rates of divorce went off the charts. Moreover, the theory of companionship marriage also made homosexual marriage thinkable in ways that was never were before (even to homosexual themselves!). Because companionship and romance took over as the rationale for marriage and people of opposite genders can obviously have these experiences as easily as people of the same gender, why not gay marriage?
This is why the gay marriage argument is so powerful in our context, even though at best it's an exercise in the logical fallacy of "begging the question." In other words, what advocates for gay marriage already assume is that homosexual relationships are equivalent to heterosexual relationships, and consequently, denying homosexuals the ability to marry is an act of discrimination. Gay marriage is a a matter of "marriage equality." Nevertheless the question remains: Why can the advocates of gay marriage assume that there is this equivalency and appeal to it? Because they assume that marriage is a public ratification of our subjective feelings about another person- i.e., companionship marriage. Since all heterosexuals in our society already assume this, such an appeal works. If one, for example, believes that marriage is an Order of Creation, and tied to specific heterosexual activities, then the argument doesn't work. Also, if one assumes that marriage ordains certain goods that are tied to the gender diversity of the persons involved and that these goods remain good irrespective of the subjective feeling of the partners, then the argument also falls apart.
But almost no one does, and that's why the gay marriage debate is not winnable for Christians in this society: We've already abandoned the correct understanding of marriage a long time ago. We can't appeal to a model of marriage that even conservative Christians unconsciously don't ascribe to.