Talk about being left at the altar: Just as gays are finally to be allowed into the house of marriage, everyone else is departing.

In the very week that an Ontario court found that Canada's historic definition of a marriage as the union of a man and a woman was unconstitutional, preparing the ground for a Supreme Court challenge that will almost certainly end in the same result, Statistics Canada reported that a majority of couples under 30 are now choosing to live common-law, rather than marry.

There are now nearly 1.2-million common-law couples in Canada, four times as many as there were 20 years ago. Partly in consequence, the fertility rate has dropped to an all-time low, since common-law couples are less likely to stay together or to have children if they do. But with proportionately fewer couples choosing to marry, the number of children born out of wedlock, as a share of all births, is rising.

And why not? A stream of decisions, issuing from both the courts and the legislatures, have steadily eroded whatever legal distinctions may once have existed between marriage and common-law unions. Indeed, in some ways living in sin may now be the smarter choice, whether in terms of liability for taxes or eligibility for benefits.

So the formal legal status to which gay couples have long aspired -- some of them, at any rate -- is one that has been largely emptied of meaning. As it has been said, about the only privilege still reserved to lawful matrimony is the right to get divorced -- a right to which an ever increasing number of married couples are resorting.

Conservatives will be inclined to view gay marriage as simply another in the long list of indignities visited upon the institution, if not somehow responsible for the rest. The truth is altogether different. It was in a tortuous attempt to avoid sanctioning gay marriage that our legislators were driven to erase all distinctions between marriage and common-law.

Consider the case of Bill C-23, "An Act to modernize the Statutes of Canada in relation to benefits and obligations." Passed two years ago, the legislation was supposed to bring federal laws into conformity with several Supreme Court rulings, which held that denying gay couples access to spousal benefits was discriminatory.

But the government was caught in a dilemma. How could it, on the one hand, claim to be eliminating discrimination against homosexuals, and on the other, leave intact the biggest single act of discrimination, at least in symbolic terms, that of reserving the "honourable estate" of marriage to heterosexuals?

Yet as radical as it was in altering spousal benefits to include gay couples, it dared not extend this quest for equality all the way to the definition of marriage, and for the same reason: while the public was sympathetic to the first cause, it was wary of the second. Instead, another solution was devised. Along with treating all unmarried couples alike, regardless of their sexual orientation, the legislation extended substantially all of the benefits and obligations of married couples to any "conjugal relationship." Even the definition of common-law was relaxed, from three years' cohabitation to one.

Rather than put gays on an equal footing with straights, in other words -- in marriage as in other areas -- the legislation equated marriage with shacking up. Had the government chosen to legalize gay marriage, it could have easily justified maintaining a separate legal status for married couples, as opposed to common-law: There is, after all, a world of difference between a formal commitment to live as one "till death do us part" and the mere fact of having shared a bed for 12 months. Instead, it sacrificed the supremacy of marriage to preserve a specious equality, even as it left a flagrant insult to gays on the books.

And now even this vandalism will be for naught, as the government must have known it eventually would be: It will have to change the definition of marriage anyway. Good. Maybe with the issue of discrimination against gays out of the way, we can get back to discriminating in favour of married people.

The conservative case for gay marriage has until now focused on what it might mean for gays: that with the social legitimacy the institution implies, gays might be encouraged to, as it were, "come in," adopting the behavioural norms of society at large. The promiscuity to which so many gay men, in particular, seem to be drawn, may in part be a function of their marginalization.

But there's an argument to be made that gay marriage may in fact be good for the institution of marriage. Perhaps, seeing the determination of a minority among us to be admitted within its confines, we will be more appreciative as a society of its virtues. Perhaps we can learn something from them.

The house of marriage is in some disrepair. Maybe they can fix it up a little.