Faith and Reason: Christian love but no procreation: This week we start a new series, exploring Christian answers to the question Why Have Sex? The first writer is Diarmaid MacCulloch, who lectures in church history at Bristol.

ORGANISED religion always takes an interest in sex, usually in order to tidy people's sexual lives into some easily managed pattern. This will no doubt be the aim of the Vatican's renewed foray into sexual morality, a document - drafts of which are already circulating - to be entitled Veritatis Splendor, 'the splendour of truth', or 'the brightness of reality'.

But what is reality amid the sweaty delights of sex? The Vatican will repeat that reality is to be found in God's command to procreate the species. Good sex has the potential to produce children; bad sex is everything else. So bad sex includes heterosexual acts involving contraceptives; masturbation; gay sex acts of all sorts. On the propagation of this fatally simple idea among Roman Catholics, many of the world's miseries now depend: the population explosion, the spread of Aids through unprotected sex, the victimisation of gay people.

The equation of sex and procreation remained convincing for centuries because contraceptive devices were expensive, unreliable and even more comic in appearance than they are now. However, over the last century, contraception has transformed the way in which human beings use and experience sex. Sex has always been fun: readily available contraception has shown that the fun can be detached from the possibility of having children. The Christian tradition is now faced with the reality that pleasure and procreation are two separate purposes of sexuality, and many parts of the Christian Church, including the Vatican, are baffled and angry at the results.

How can the Church cope? A first step would be to recognise the splendid truth that its traditional views on sexual intercourse were filched from non-Christian sources. Christianity is not just an ecstatic cry that Jesus is Lord. It is a complex system of thought with two main strands: Jewish and Greek. Of the two strands, the Greek has made the running for nearly 2,000 years. Even though Jesus was a Galilean Jew and probably had little contact with Greeks, the enthusiasts who wrote up his life and discussed his ideas took Christianity far from its Jewish roots. Most of their potential audience had a Greek cultural background, and in trying to make Greeks understand the message, Christianity absorbed the culture which it was trying to capture.

So rather than Jesus Christ, or even Paul of Tarsus, a sequence of pre- Christian Greek philosophers set the tone for Christian discussion of sex. In particular, Aristotle's wholly wrong-headed discussion of human biology lies behind the Vatican's obstinate urging of sex-for-procreation. Aristotle asserted that male semen was the most important factor in the conception of a child. Male seed contained the entire foetus in embryo: a woman's function was simply to act as an incubator while the child grew. Even practising doctors agreed: the standard Roman authority Galen said that there was no difference between sowing seed in the womb and sowing the earth. Hence the superstitious value attributed to semen in ancient discussion of sexuality: to produce semen in any other context but procreation was to kill a human being.

Christian theologians in late second-century Egypt took up the theme: 'to have sex for any purpose other than to produce children is to violate nature', said Clement of Alexandria. It does not inspire confidence in Alexandrian judgement on matters sexual that Clement's successor, Origen, is said to have castrated himself because he regarded his sexual organs as a source of moral danger. However, these views on sex were so influential in the Church that one can call the equation of sex and procreative potential the Alexandrian rule.

The rule was repeated with enthusiasm by Thomas Aquinas, who did so much to make the Church of Rome see the world through the eyes of Aristotle. And so matters in the Vatican rest from the 13th to the 20th century. Its celibate theologians apparently do not now adopt Origen's desperate measures, but the splendour of modern reality has yet to inspire enthusiasm in them.

There are other Christian possibilities. In the Protestant Reformation, many theologians married, and began discovering that (with a bit of luck and hard work) marriage could be fun. England's first married archbishop, Thomas Cranmer, said as much in his revamped marriage liturgy in 1549: having dutifully recited the traditional liturgical reasons for marriage (procreation and avoiding fornication), he proclaimed with first-hand knowledge that marriage existed 'for the mutual society, help and comfort that the one ought to have of the other'.

It was no coincidence that the Anglican Communion was one of the first churches to recognise the new reality that contraception was here to stay. Anglican bishops were still expressing 'repugnance' at the very idea of contraceptives when they met at the Lambeth Conference of 1908, but as early as 1930 they accepted that there could be 'a morally sound reason' for practising sex in marriage which could not lead to procreation.

The 1930 Lambeth declaration is quite clear that pleasure and procreation can be separated, but it still firmly places that pleasure within the bounds of traditional marriage. The implications of the contraceptive revolution are far wider. Do the Churches have anything but finger- wagging criticism or dire warnings to offer same-sex partnerships? or heterosex partnerships which draw back from the traditional marriage model?

The Christian faith can draw on its own resources, once Aristotle and the Alexandrian rule have been banished to the theological lumber-room. Of course there are dangers in any new arrangement in human behaviour, but one does not avoid the challenge by rushing back to Aristotle's skirts. Christianity is a love-poem. It should not be afraid of love, even when the love seems dangerous and unfamiliar. Christianity has danger built into it. One of its central liturgical symbols in the eucharist is alcohol: wine which is both an icon of life and fun and an icon of death and destruction. The Bible is a library on the subject of love. The Christian story begins with God creating the world out of his love, and seeing that it is good: the Jewish love of life fills the Old Testament with an exuberance which is sometimes positively embarrassing to the Church.

But the Christian love-poem is incomplete without its story of danger embraced and overcome. God enters flesh and in his infinite love, takes on human suffering and imperfections, but also human dignity and joy. When Jesus is on the Cross, he is 'reigning in triumph on the Tree', a symbol both of the infinite possibilities and the infinite dangers of our earthly relationships - for it was love which brought him to the Cross.

Conservative Christians often mock liberal discussions of Christian love as wishy-washy avoidance of serious moral issues. But there is nothing wishy-washy in taking the danger at the heart of Christianity seriously. Sexual relationships are all the more dangerous for having experienced the contraceptive revolution of the last century: but in their untidy modern reality, they are just as much an icon of Christ as any neat moral scheme from the ancient world.