On the purpose of this voluntary union, the Book of Common Prayer is correct on one of three counts.
Who "owns" marriage? The Archbishop of York thinks that he does, or at least that the church, rather than the government, is its custodian. "It is set in tradition and history and you can't just [change it] overnight, no matter how powerful you are," he warned ministers in an interview with the Telegraph last month. Lynne Featherstone, the equality minister, hit back at the weekend with a piece of her own in the Telegraph. Marriage, she asserted, is owned by "the people"; and that therefore the government, whose job is "to reflect society and to shape the future", has a perfect right to extend the scope of legally-recognised marriage to gay couples.
It's significant that traditionalists are now making what looks like a last stand on the word "marriage" itself, having largely accepted the existence of civil partnership, a phrase invented partly to convince traditionalists that the new status was not the same as marriage. In fact, the distinction is largely a semantic one. There are, perhaps, genuine fears that if the word "marriage" was used, clergy would find themselves faced with legal action if they refused to officiate at gay weddings. But that doesn't seem to have been what Sentamu had at the forefront of his mind when he accused reformers of "trying to change the English language".
Appealing to the "naturalness" or "universality" of exclusively heterosexual marriage, as Dr Sentamu did, is a clever rhetorical strategy, once used by defenders of slavery or racial discrimination. But in truth, the mainstream modern concept of marriage as the romantic union of one man and one woman is neither natural nor universal. It's a fairly modern construct, dating back not much further than the middle of the last century. Rather than being immutable and God-ordained, it is an artefact of developed capitalist societies.
Of course, all known societies have had something that anthropologists recognise as marriage, and in almost all known cases it is a heterosexual institution. The religious conservatives are right about that at least. There are a few exceptions that prove the rule. In some parts of Africa, for example, a woman may take on the attributes of a man, including a wife. She becomes a "female husband" and the legal father of any children born to her wife. But such arrangements are mechanisms to preserve family property (typically when there is no male heir) rather than romantic unions; both partners are expected and indeed encouraged to take heterosexual lovers to satisfy their physical needs.
This points to the reason why legally-defined marriage has, until very recently, been an exclusively heterosexual business. What distingugishes marriage, anthopologically speaking, from other forms of attachment both heterosexual and homosexual is not the relationship of the couple (assuming that they are "a couple") but securing the legitimacy of children.
In most societies, marriage also has an important secondary function of creating bonds between wider kinship groups. In an agricultural society, where land forms the basis of the economy and there is relatively little mobility, either social or geographical, marriage has been the bedrock of society for entirely practical reasons. And these practicalities have produced forms of marriage that romantic modern Westerners, brought up on the myth of the couple, tend to look upon with suspicion or horror: polygamy, arranged marriages, child marriage, levirate (the automatic marriage of a widow to her late husband's brother), dowry systems and so on.
How we got from there to here is a long and complex process. Christianity certainly played a part in it, so it's worth looking briefly at how the church traditionally defined marriage.
According to the Book of Common Prayer there are three purposes for marriage. They are, in order (presumably) of importance: children, sex and companionship. Marriage provides a legitimate and secure setting to raise the next generation, "to be brought up in the fear and nurture of the Lord" as the marriage service has it. This has always been conservatives' favourite argument for preserving the special status of marriage. Nevertheless, a high proportion of children are now born to cohabiting couples and the social stigma on births outside wedlock has largely gone. As for the second reason, ("a remedy against sin, and to avoid fornication") I would only observe that the number of people who insist on marriage as the only proper setting for sex is now fairly small, and the number who rigorously practice such an ideal is considerably smaller still.
Instead, the stress now falls heavily on the third of the traditional marriage service's stated aims, companionship, what the Prayer Book rather beautifully describes as "the mutual society, help, and comfort, that the one ought to have of the other, both in prosperity and adversity." Marriage is now less about creating a family than about celebrating and cementing the relationship of the couple. It is about love and commitment; and it is about two people (or, in an increasing number of cases, about the two people and the children they already have).
Marriage is now an essentially voluntary union that expresses the couple's commitment to one another, to sexual fidelity (usually), to children (optionally), to their religious traditions (sometimes) and to social expectation (decreasingly). To conservatives, marriage serves as a source of stability. The individual commitments of couples still form, they would argue, the basis for a stronger society as a whole. But that is not because the couples are heterosexual; it is because they are couples. And the past few decades have revealed, to the surprise of many, that gay couples can be as traditionally-minded and conventional as straight ones. Why else would they want to get married?