Classically defined, marriage is the union of a man and a woman for life. Not so long ago, it was understood to be the mutual undertaking of a free man and a free woman to live with each other during their joint lives "in the union which ought to exist between husband and wife".

Today, it would be outrageously impolitic to define marriage in these terms. Yet, no one should forget that traditional marriage was not open to slaves. How could it have been? Slaves were real estate.

"Things" just don't properly marry, do they? The Church used to say that this "ought-to-exist" union was characterised solely by the procreative act. There is recent revisionism of this long-established feature. Marriage is now said to embrace ideas not only of childrearing but also of "marital friendship".

Marital friendship is a cute, evasive cue for notions ranging from tenderness to erotic love. Notwithstanding its religious overtones, marriage is secular in modern law.

Contemporary law does not purposely subscribe to any particular notion of revelation. So, in our part of the world, secular marriage is the union of a man and a woman. Full stop.

The indissoluble and procreative aspects of marriage are helpful, perhaps even aspirational, features. They're just not essential.

Adoptive parents, senior citizens, single mothers and fathers, men and women in second marriages, couples who cannot have children or couples who simply don't want children should all be thankful that secular law ? the law of pensions, housing allowances, survivorship benefits, and preferential tax treatment ? has jettisoned procreation as the sole qualifier.

Some countries have lots of different types of marriage. India is one example, Bali another. In India, marriage is treated as part of the personal law of the parties.

Under Hindu law there are two orthodox forms of marriage. The Hindu Marriage Act regulates these marriages. Muslim marriages are governed by the Muslim Personal Law (Shariat) Application Act. There are also statutory marriages for Parsis, Sikhs, Jains, and Christians.

Finally, the Special Marriage Act governs all others, except Jews whose marriages are regulated by custom. In India marriage has a distinct religious focus.

On the other hand, in France and Italy, marriage has a secular focus. Couples must be civilly married, and civil marriage is the only officially recognised form.

In these countries and in many others, marriage is a civil union but not a "civil union", that new home for intimacy.

"Civil unions", or domestic partnerships, or registered partnerships, or civil partnerships are often called "gay marriages" as if all civil unions were open only to gay men and women.

This may well be the case in the United Kingdom or in Vermont, but it is not the case in Quebec or California.

Initially, the Quebec government introduced civil union legislation for same-sex couples only. The draft legislation was then amended to include opposite-sex couples, and this legislation is now law.

Similarly, the California domestic partnership is open to qualifying opposite-sex couples if one or both partners are over the age of 62.

In 2003 California Governor Gray Davis hailed the legislation as "a triumph for lesbian and gay rights and senior citizen rights". In Germany and all the Scandinavian countries, registered partnership is only available to gay couples. In France, Belgium and the Netherlands, all are welcome.

Domestic partners are adults who have chosen to live in an intimate and committed relationship of mutual caring. They "express their free and enlightened consent to live together," as the Quebec lawmaker has provided.

This sounds a lot like marriage. So much so that in Quebec and Vermont there is absolutely no difference between the rights and duties of marriage and civil union, at least within their respective constitutional contexts.

On the like nature of the two institutions, the Quebec legislation is telling. Civil union partners are called spouses.

Moreover, the legislation specifies that the following language be used by the presiding officer: "By virtue of the powers vested in me by law, I now declare you united in the bonds of civil union. You are now legally united. Allow me, on my own behalf and on behalf of all those present, to offer you our best wishes for your happiness."

Does this sound familiar? Disparagingly, some jurisdictions register civil unions in a manner more akin to the renewal of a driver's licence than to the celebration of an intimate bond.

The Quebec legislator has also provided special rules for civil union spouses to "upgrade" ("downgrade", some might say) to marriage.

Why would this be so when marriage and civil union in that province are indistinguishable? It would seem that there is some yearning for the institution of marriage that cannot be rationally expressed in law but which nonetheless responds to the gut and which testifies to a deep-seated desire for social acceptance.

If marriage today, for civil purposes, is the lawful union of two persons to the exclusion of all others, and civil union is the same thing, then what distinguishes civil union from marriage?

In part, it may be that marriage and religion cannot be disentangled. Their histories are inextricable, and some citizens do not care to be part of that churchy tradition. In this light, civil union can be regarded as a fresh start for intimate relations.

It has no history of abuse nor, admittedly, any of happiness. In a way, it is marriage reinvented for people who like to invent.