The bishops and cardinals who have been so furiously, and a little fearfully perhaps, condemning the plans to allow gay people to get married say they are opposed to the reform because marriage can only be between a man and a woman.

But what the biographies in Gay Life Stories demonstrate is that it's not quite as simple as that.

The author of the collection, Robert Aldrich, calls the life stories, in a nice cheeky use of religious language, a "congregation" and the effect of it is that you start to notice the collective lessons of their lives. One of the clearest is that relationships and marriage are not as easily defined as the bishops might think.

Some of the early biographies are the most enlightening in this regard. The earliest is from around 2400BC and tells the story of Khnumhotep and Niankhkhnum, whose tomb was uncovered in Egypt in 1962. Not only were the two men buried together, carvings in the tomb show them embracing, holding hands and staring into each other's eyes. They may have been friends, they may have been brothers, they may have been in a same-sex relationship. The point is that it doesn't matter – what the story seems to show is that the boundaries between affection, intimacy, sexuality and commitment that the bishops seek to nail down and keep in place, are much more adjustable and changeable than they realise, or would want.

On the question of marriage, some of the other biographies in the book are even clearer. The biography of the 17th-century Chinese poet Chen Weisong, who lived with another man, Xu Ziyun, includes the fact that male marriages were celebrated in the province of Fujian. Ceremonies were also devised by the Eastern Church for joining two men together in rituals that were remarkably similar to weddings.

In other words, the religious critics of gay marriage can argue that only Christian traditions should apply in Britain, but they cannot argue that for thousands of years marriage has only ever been defined one way.

It would be wrong, however, to reduce Gay Life Stories to a mere rebuttal of the arguments against gay marriage because it is more interesting than that. Many of the stories have a heroic element – particularly those of the French writer Andre Gide, the Victorian landowner Anne Lister, and the poet Edward Carpenter – and that is needed to balance the stories that inevitably have tragic elements: violence, Aids, homophobia.

Aldrich's writing helps to maintain this balance as well as bringing out another important point: not all gay people are the same shape and size as the popular stereotypes. There are different social and personal experiences, different political approaches, and gay men and women are called to a whole range of professions. Most of Gay Life Stories demonstrates this, but particularly the story of the French general Hubert Lyautey. Now that so many Western countries have ended their ban on gays in the military, men and women in a similar position to Lyautey no longer have to hide. If we get a new edition of Gay Life Stories in 100 years, it may well contain many more soldiers.

But what of the 4000 years the book actually covers? What do they prove? Probably that, even though some countries have gone from tolerance to intolerance, the direction of travel on gay equality is, on the whole, forward. There is more acceptance, more openness; there is a brighter light.

Gay Life Stories

Robert Aldrich

Thames & Hudson, £19.95