It is possible to be well-disposed towards civil partnerships and open to celebrating them liturgically, yet to feel unease about two points that often occur in discussion about them (for example, in Mark Vernon's article, Comment, 11 November).

First, we may wonder whether heterosexual marriage really is so inherently flawed by possessiveness that marriage has not been about friendship. Second, we should certainly question the assumption that rites for friend-making existed in Western Christianity based on Aelred of Rievaulx's theology of friendship: this claim lacks historical foundation.

"Marriage is the queen of friendships," wrote Jeremy Taylor in 1657, and Aelred, whose works were unknown to Taylor, would readily sympathise. Aelred rewrote Cicero's dialogue On Friendship for the mon-astic setting of Rievaulx in 1160, Christianising it from the Bible and early church writers.

He concludes from Genesis 2 that equal friendship was the intended state of humanity. If Eve was taken from Adam's side ("de latere"), it shows that human beings are "collateral, and that there is in human affairs neither a superior nor an inferior, a characteristic of true friendship". The Fall introduced dissension; but among the good this mutual charity still pertains, he argues.

Theological discussion of the relational aspect of marriage has focused not on possession, but on the equality of the sexes in wisdom and intellect, spiritual gifts, and authority. Paul read Genesis differently from Aelred, placing woman beneath man as her origin or head: here lies the nub of the debate.

Aristotle opined that true friendship based on virtue could exist in marriage, while nevertheless assuming that man is inherently superior to woman, and hence has overall control, delegating to his wife only those things for which her own virtues fit her.

Thomas Aquinas praised marriage as exemplifying friendship to the highest degree. Whatever his Aristotelian biological ideas, he affirmed the spiritual equality of men and women in that their souls (mind, "mens") both alike bear God 's image. Jeremy Taylor still took it as axiomatic that in this "queen of friendships" man as the wiser partner must ultimately be in charge. There is, then, an ancient tension in marriage - between the equality of friendship and presumed sexual inequality.

Marriage is not an inherently unequal partnership; it has simply reflected the presumption of inequality that has been prevalent in (fallen) human society, and hence crept into theological anthropology. To the extent that wifely obedience was predicated on the presumed inferiority of women, it naturally falls away when inequality is removed - either by an Aelred or by modern critique. Where there is a presumption of equality, marriage becomes unequivocally "friendship plus sex", and family life can be built on that basis.

Possession is an issue distinct from that of authority, although capable of being confused with it. The custom of "giving away" suggests the perpetual legal minority of women, which is an equality issue. Its overtone of some kind of legal possession is, however, wholly cultural (Old Testament culture included), and boasts no Christian theological underpinning.

Possession in any quasi-legal sense would indeed seem to exclude friendship, although it might be worth reflecting that Aristotle suggested one could be friends with one's slave qua human, but not qua slave, in which capacity he is a mere tool. This sounds like a difficult balancing act, but perhaps not unlike the situation obtaining in some  Christian marriages (in the past, one hopes).

Mr Vernon and others have pointed to observable differences between heterosexual couples, who become inseparable social units, and gay couples, who are happy to do things separately with other friends. There must be a spectrum of behaviours here - but one would like to know how far Christian gay couples would agree, and whether they would testify to the "becoming one flesh" that issues in a desire not to be separated.

If the marital physical relationship makes two souls in one body, that of marital friendship makes the classical "one soul in two bodies". Marriage should be complete in both senses. Such one-ness has nothing in common with legal possession. It transcends possessiveness, and reveals how rightly, deeply, and equally each possesses the free gift of the other.

Did a liturgy of same-sex friend-making exist in medieval Western Christianity, based on Aelred of Rievaulx's theology? This claim is misleading in several respects. Aelred did not suggest any vow, and his dialogue Spiritual Friendship remained virtually unknown for centuries.

There was an Eastern (Balkan) rite for adelphopoiesis, "brother-making", which does emphasise friendship; and there are reminiscences of sworn brotherhood in the Western Middle Ages. The Western material is well presented in Alan Bray's The Friend (Chicago University Press, 2003).

Yet, as Bray concludes, such vows of brotherhood took place outside the mass, were rooted in contexts very different from our own, and were undertaken for a variety of reasons. They cannot simply be read as parallels for modern civil partnership. It is unlikely to be helpful to build for the future on tendentious interpretations of the past; and we are now in a genuinely new situation.

Friendship should undoubtedly be a common theme in marriage and civil partnerships. In developing its new rite of blessing, the diocese of New Westminster has drawn on covenant theology. In this, as in marriage, friendship is affirmed by an act of solemn commitment.

The Revd Dr Liz Carmichael is chaplain and tutor in theology at St John' s College, Oxford, and the author of Friendship: Interpreting Christian love (Continuum, 2004).