Philip Hensher: There's more to marriage than a contract between a man and a woman

The understanding of marriage has altered considerably over time, and across cultures.

The ancient Greeks prosecuted those who married beneath them, those who married too late, and those who did not marry at all – it was compulsory, like voting in Australia. In many parts of the world, partners in marriage may have met each other infrequently or not at all before their commitment; the decision to marry could rest on such factors as shared diet or the alignments of the stars. It might not be a decision made by the couple themselves, but by their families without consultation.

Elsewhere, marriage is not even viewed as something undertaken by adults. In parts of the world, it is common for girls to marry before puberty – eight is not unheard of in Yemen or Saudi Arabia, and there are cases of babies being married to adults, although in these cases, "irrevocably promised in marriage" is usually closer to our understanding. We in 21st-century Western culture view marriage as founded, above all, on sexual compatibility. That would have been very surprising to previous ages, where the mariage blanc was a perfectly respectable possibility. In France, it is legally possible to marry a dead person, so long as the preliminary steps have been taken before the participant's death; in China, for reasons beyond the reach of all but the anthropologist, two dead people may be taken through a marriage ceremony. In Sudan, in 2006, a gentleman called Charles Tombe was required to marry a goat and pay a dowry of 15,000 dinars, after being discovered having sex with the animal.

In the past, definitions of marriage which now appear quite innocuous to us aroused furious passions among law-makers. It took nearly 50 years of campaigning from the 1860s onwards until widowers were permitted to marry their sisters-in-law. In an age when unmarried women often helped to bring up the children of their dead sisters and keep house for their brothers-in-law, this prohibition must have caused great heartache until the passing of the Deceased Wife's Sister Act, 1907.

Marriage, far from being a permanent, unchanging and sacred institution, is a concept which changes enormously across cultures and through time. None of this ventures on the vast territory of different marriage customs, but only on the fundamental concept of what marriage means within a family or a society. My husband remembers his two great-grandmothers: the two widows of the last man in his family to marry polygamously, at the time something thought respectable and normal. We think that a very peculiar idea, and strain to comprehend the idea that they were great friends and lived together harmoniously. What a strange idea of marriage, we think. But then they might have looked at us, and thought it just as strange for two men to get married, as he and I did in the spring of last year.

Currently, same-sex marriage with exactly the same status as heterosexual marriage is enshrined in the laws of seven European states, three in the Americas, and one African – South Africa. Eighteen other states, including the UK, offer some form of registered partnership. These vary in status: the UK's civil partnership offers almost all the legal benefits of marriage, apart from the fact that when Stephen Fry gets a knighthood, his partner will have no courtesy title to bandy about. Others appear to be chiefly generalised expressions of goodwill, offering a notary, a desk and a piece of paper to sign.

Nearly a dozen US states offer some form of legal recognition of same-sex unions. The case of gay marriage in California is a curious one. In November 2008, as part of the state elections, a ballot proposition was passed stating that "only marriage between a man and a woman is valid or recognised in California". Previously, Californian courts had held that same-sex couples had a constitutional right to marry, in accordance with the well-known American principle of the right to the pursuit of happiness.

This proposition, known as Prop 8, held until this week. Judge Vaughn Walker overturned it. On the most abstruse legal grounds imaginable, he further cast doubt on whether the opponents of the proposition had the legal grounds to mount an appeal, stating that because of various inward considerations, only the governor and the attorney general of California could do so. Governor Schwarzenegger declined to do so; the ban was lifted from 12 August, and the first marriages will take place from 18 August.

In the rest of the world, amusement has been expressed over the fact that this is taking place in California. The chic salons of the movie industry have vocally promoted the right of gay people to marry in public; as many people have observed, it's curious that when it comes to their star actors, they seem so reluctant to tell the truth about what everyone knows to be the case. I don't expect to see a major Hollywood actor at the height of his career marry, or even allude to the existence of, his long-term boyfriend for another two decades.

It's still more amusing that same-sex marriage in California appears to have come about not through moral principle; not through simply thinking it's the right thing to do; not even by listening to the will of the people who would want to take advantage of the new law. Instead, it seems to have come about by a ruling about the right to appeal. Walker said: "When proponents moved to intervene in this action, the court did not address their standing independent of the existing parties. This court has jurisdiction over plaintiffs' claims against the state defendants pursuant to 28 USC S 1331. If, however, no state defendant appeals, proponents will need to show standing in the court of appeals."

That is not the Gettysburg Address of the gay marriage movement; it will not ring down the annals of Californian history, and I doubt children will recite it as a defence of American freedom any time soon. Nevertheless, it does the job. Its principle seems less resonant than the one which previous campaigners have relied upon, that it ought to be possible for people who love each other to make a permanent commitment which will protect their rights and responsibilities towards each other. But Walker's pedestrian words conceal a truth just as important. The happiness of individual people's lives should not be placed at the wilful pleasure of well-funded lobbying campaigns. There should be some security in these matters, and once you have married, you should not have to fear that your marriage will be forcibly dissolved by an advertising campaign mounted by ProtectMarriage.com, or, when it fails, by some other lobbying group stepping up to the plate.

The Deceased Wife's Sister controversy of our day is, surely, same-sex marriage. Can anyone seriously suppose that in 50 years, anyone will care about these issues? Will not any sane person wonder about the motives of those who sought to prevent the private happiness of individuals quite unknown to them? California is not in the vanguard of civil liberties in this area. A dozen countries in the world are in advance of the freedoms it offers to its citizens. But there is no reason why it should not represent the future in the eyes of much of the rest of America, and, indeed, the world. Think of the terms on which your great-grandparents' marriage was conducted, and ask yourself whether the changing concept of what marriage includes and refers to should stop here. Or whether it's at all likely to.