One of the favourite arguments of those opposed to same-sex marriage is the idea that the institution of marriage embodies the "complementary" nature of men and women. Just as (they would say) marriage and civil partnership are "equal but different", so are the genders. The fact that the argument invariably comes from those espousing what they like to call the "Biblical" view of marriage (conveniently skirting over all those polygamous patriarchs in the Old Testament, but never mind) is apt to raise suspicions that what they really mean is that women belong at home in the kitchen while their husbands are out winning the bread in appropriately manly ways. But I doubt it's necessarily as reactionary as that, at least not in the minds of many of those putting it forward - liberal Anglican bishops, for example.
Speaking yesterday in the House of Lords , the Bishop of Leicester offered a superficially convincing modern twist on the idea.
I could not help noticing in the debate in this House on International Women's Day the underlying assumption that women bring a special quality to the public square and that the complementarity of men and women is what encriches and stabilises society. Yet, in the realm of public discourse, assertion of sexual difference in relation to marriage has become practically unspeakable, in spite of the fact that it is implicity assumed by most people in the course of everyday life. Equal marriage will bring an end to the one major social institution that enshrines that complementarity.
He's got a point. Not, I think, about the threat that equal marriage would allegedly pose to "sexual complementarity": inasmuch as that matters, it matters to the individuals concerned, so that equal marriage could only threaten "complementarity" if it magically turned everyone gay. Where the bishop might just be on to something is in his identification of the importance that society still, indeed increasingly, attaches to gender difference. There does indeed seem to be an "underlying assumption" that men and women are essentially different beings, and arguments for expanding the role of women do often come down to enumerating the unique gifts that women are said to bring. "Men are from Mars and Women are from Venus," as the ever-popular relationship manual assures us.
In the Church of England itself, the case for appointing women as bishops has been made not just on simple grounds of equality (here are some excellent potential bishops who just happen to be women) but by stressing the special qualities of women - the "maternal" quality of their pastoral care, for example. The same is true of politics and the world of business, where it has become commonplace to blame macho attitudes for wars or the banking crisis.
And what special qualities do women bring? Invariably, they turn out to be the very attributes that have always been considered quintessentially feminine and that were once trotted out as reasons why women's proper place was in the home. The contribution of women is celebrated, even by many feminists , on grounds of difference (though it is a difference disguised as superiority) - women are assumed to be more consensual, less competitive and aggressive, more concerned with nurturing and supporting others. One side-effect of such thinking is that women who are none of these things (the late Baroness Thatcher springs to mind) are apt to be denigrated as un-feminine.
The potency of the idea of sexual difference is remarkable, given the speed with which women have joined professions once considered a male preserve, from the legal profession to the military, where they operate increasingly (though not yet fully) on equal terms. Barely a day goes by with out some new scientific study confirming the existential difference between men and women, where a statistical correlation is interpreted as an iron law of biology and the interplay between biology and culture is never examined. Women may have more economic independence than ever before, but that hasn't prevented the increasing commercial and cultural enforcing of gender norms, with Tesco categorising chemistry sets as boys' toys  and Disney reimagining the tomboyish Merida as a slim-waisted princess . Gender binaries rule.
So the bishop of Leicester really has very little to worry about. In the days when a woman had to promise to obey her husband and all her property became his the bishop might have had a stronger case. Such an institution would indeed have provided a most inappropriate model for same-sex relationships. The modern basis of marriage, on the other hand, is partnership and equality rather than innate difference. Far from being the sole redoubt of sexual "complementarity", in a world seemingly more convinced than ever that men and women come from different planets marriage has in fact become a challenge to it.