Archbishop Rowan Williams has tried to take the issue of gay marriage off the table at the Lambeth Conference, which begins in three weeks. But the celebration of a gay relationship at one of London’s oldest churches last month, and the well-publicised gathering of anti-gay Anglicans in Jerusalem this week, suggest the controversy must eventually be faced squarely.

Conservative Christians say opening marriage to gay couples would undermine an immutable institution founded on divine revelation. Archbishop Henry Orombi, the excitable primate of the Church of Uganda, calls it blasphemy. But, theologically, support for same-sex marriage is not a dramatic break with tradition, but a recognition that the church’s understanding of marriage has changed dramatically over 2,000 years.

Christians have always argued about marriage. Jesus criticised the Mosaic law on divorce, saying “What God has joined together let no man separate”, but even that dictum appears in different versions in the Gospels, and was modified in the letters of Peter and Paul. The Christian tradition includes the ecstatic sensuality of the Song of Songs and Paul’s teaching that marriage was a fallen estate, useful primarily in saving those who could not be celibate from fornication.

This tension is indicative of the church’s long struggle to reconcile the notion that sexuality is a gift from God with its deep suspicion of the pleasure of sex. As the historian Stephanie Coontz points out, the church did not bless marriages until the third century, or define marriage as a sacrament until 1215. The church embraced many of the assumptions of the patriarchal culture, in which women and marriageable children were assets to be controlled and exploited to the advantage of the man who headed their household.

The theology of marriage was heavily influenced by economic and legal considerations; it emphasised procreation, and spoke only secondarily of the “mutual consolation of the spouses”. In the 19th and 20th centuries, however, the relationship of the spouses assumed new importance, as the church came to understand that marriage was a profoundly spiritual relationship in which partners experienced, through mutual affection and self-sacrifice, the unconditional love of God.

The Episcopal Church’s 1979 Book of Common Prayer puts it this way: “We believe that the union of husband and wife, in heart, body and mind, is intended by God for their mutual joy; for the help and comfort given one another in prosperity and adversity; and, when it is God’s will, for the procreation of children and their nurture in the knowledge and love of the Lord.”

Our evolving understanding of what marriage is leads, of necessity, to a re-examination of who it is for. Most Christian denominations no longer teach that all sex acts must be open to the possibility of procreation (hence, contraception is permitted). Nor do they hold that infertility precludes marriage. The church has deepened its understanding of the way in which faithful couples experience and embody the love of the creator for creation. In so doing, it has put itself in a position to consider whether same-sex couples should be allowed to marry.

Opponents of gay marriage may raise other objections — that it is unsuitable, for instance, to raise children with two mothers or two fathers. I believe these arguments are easily refuted, but they are arguments about effective social policy, not sound theology. Christians who want to deny others the blessings that they claim for themselves should not assume they speak for the Almighty.

John Bryson Chane is Bishop of the Episcopal Diocese of Washington DC, and a member of the Chicago Consultation, which works towards the full inclusion of gays and lesbians in the Anglican church.