If Damian Thompson is to be believed, and I think he makes sense, 'gay marriage' is on the British political agenda because, well, there are votes in it. Not lots of Christian votes (though there will be some) but secularist, atheist post-modern young adults' votes, of which there are an increasing number and to which David Cameron's antennae are tuned. Accordingly, key political figures are lining up with key cultural figures in a kind of trans-Westminster/media/cultural elite 'three line whip' to ensure it happens. Woe betide the church (Anglican or Catholic) standing in the way of this change.
Something similar is at work in parts of the States, including a bit of a push at federal level, but the States is a little more complicated (e.g. because such changes get worked on at both state and federal level). It is also emerging in Oz, but not here in NZ (yet).
This political bulldozer threatens to flatten theological questions which push up from the legislative roadway (see reflection here at Catholicity and Covenant, and follow through to link to a provocative Milbank essay. I hope to come back to this soon at ADU).
At the very least Christians have a challenging path to walk in responding to this kind of political agenda.
It is quite correct and appropriate for Christians to ask for space (the same space Muslims seek re 'gay marriage') within our 'liberal' and 'multi-cultural' Western societies to explore in teaching and in practice our commitment to gender difference in marriage as foundational in our traditio-biblical understanding of marriage. However I do not put it past the confusion in the minds of leaders of Western societies in the 21st century to pick on Christians and give Muslims a free pass on this particular matter. (I have no idea whether, one day, a free pass might be given re polygamy in the West). To an extent that confusion is understandable when some Christians within our midst (e.g. the Giles Frasers in Britain) have so readily given up on the traditio-biblical understanding of marriage having gender difference at is foundation.
Conversely, it is inappropriate for Christians to act and speak as though in a post-Christendom world it nevertheless retains control of legal definition of marriage. Some speech of this kind (in my view) all too easily transmutes in the media's reporting to "Look, the church is opposed to gay people" which, frankly, is not a good look since we do not generally engage in conduct "opposed to [full in the space] people".
Nevertheless if we walk this pathway well, I wonder if we are going to be unable to avoid some conflict. One problem which I do not think is being aired in the promotion of 'gay marriage' is that inevitably a lack of gender distinction in law about marriage must change the climate in which marriage may be publicly spoken about. Speaking about marriage in a presumptive way that a 'husband' and a 'wife' might be constitutive of it - a natural way for Christians to speak - could incur a day in court because of public demonstration of prejudice against 'gay marriages'. One line in the promotion of 'gay marriage' is that it is a nonsense to say that it will further the breakdown of marriage. What is not being discussed is that the promotion of 'gay marriage' will inhibit the way we discuss marriage because there will be no legal protection for those who wish in such discussions to make a distinction between marriages between men and women and marriages between people of the same gender.
Actually, so far, even the advocates of 'gay marriage' have not been able to avoid the use of the phrase 'gay marriage' which is a sign that, in fact, Christians are right: marriage between a man and a woman' is not the same as 'marriage' between two men or between two women.
I wrote most of the above in draft form before having a read of Milbank's essay. In my view, albeit with different analysis and critique, Milbank's essay underscores what I am saying here: in Christian perspective gender difference is intrinsic to Christian understanding of marriage. What may be important about his essay as we reflect on it and discuss it (and I hope there is wide discussion of it) is that it models how Christians might publicly talk about marriage in a manner which is not against gay people.
POSTSCRIPT: Austen Ivereigh has a lovely, irenic essay, also posted on ABC.