Gleesome threesome, or menage a misery?: Can love triangles ever be satisfactory? Genevieve Fox meets three people who each have first-hand experience

FIDELITY, it would seem, has been reduced to a social anachronism. These days 37 per cent of marriages end in divorce, with 41.8 per cent of husbands and 23.2 per cent of wives citing adultery as the cause. In Robert Wright's new book, The Moral Animal, he argues that humans are genetically compelled to be unfaithful. If this is the case - and the falling marriage rate suggests the nuclear family is stranded on the rocks - then it may be about time we unleashed our libidos, and our consciences, from the constraints of one-to-one relationships.

A classic alternative is the menage a trois, practised by members of the Bloomsbury set in the Twenties, and by Neil Cassady, beat generation icon, who lived with his wife, Carolyn, and best friend, Jack Kerouac. Today Kingsley Amis shares his north London home with his first wife, Hilly, and her husband, Lord Kilmarnock.

For many, the idea of a platonic threesome is unrealistic, while a sexual three-way partnership is either morally repugnant, or the preserve of free-thinking intellectuals. Sue Pallenburg, a psychosexual therapist, cautions: 'Threesomes are a strong fantasy, but may be better off remaining that way. After the initial excitement, feelings of exclusion, of not getting enough attention, emerge. Generally, threesomes don't work because the relationship is out of balance.'

But the menage a trois is for some a practical alternative to the conventional two-person partnership. Risky, racey, it promises a form of democratic love in which the balance of power is held in check by the constantly fuelled desires of three consenting adults. Sexual variety and intellectual stimulation combine with companionship and security. The betrayal, deceit and heartbreak that accompany infidelity are replaced by understanding and honesty, safeguarded by basic ground rules. Denise Knowles, a counsellor for Relate, says she would 'neither condemn nor condone a menage a trois, assuming it suited all those involved'.

It is also a means of sexual experimentation, as we see in the new film Threesome, in which a girl and two boys - one gay, one straight - explore their sexual identities.

There are certainly thorny practical issues to resolve: who sleeps with whom, and when, or who can be seen with whom in public. Each person's emotional motivation will come into play. Does the introduction of a third person to a relationship spell not just spice and novelty, but also jealousy and insecurity? Is the man who shares himself between two women in the name of free love merely playing a selfish game of sexual power? Here, three members of three different threesomes tell their stories.

All names have been changed.