This year's Gay Pride day, which falls this Saturday, arrives with a more authentic smell of battle smoke than most years. The argument carried in the pages of this paper's Sunday sister and the billboard advertising the day's celebrations with the defiant title of Salvation are nothing compared to what has been going on in Canada and Spain in the face of the prospect of the legalisation of homosexual marriage in both states.

On Saturday the streets of Madrid are expected to be heaving with marchers out in force to outnumber the huge anti-gay marriage protest march of June 18, which saw 20 Spanish bishops and a crowd numbering... well, take your pick: 166,000 (according to the police), 700,000 (Popular Party), 1.5 million (Family Forum). Whatever the number, in a country where last year a poll indicated that 57 per cent were in favour of the legalisation of homosexual marriage, the Gay Pride march this year can be expected to be larger, more triumphant and more significant than pride marches happening elsewhere.

For exactly the same reason, the pride marches are more significant this year for all those who oppose gay marriage - particularly the large number that does so because it believes that gay marriage will have a direct negative impact on the family, placing its nature in jeopardy. Spain will be the first Mediterranean country to legalise such marriages - a highly distinctive development.

"Registered partnerships" (which grant almost all the rights of marriage, bar some to do with children, like the right to adopt) were first legalised in Scandinavia over 15 years ago; the first country to legalise marriage was Holland (2001), which marked not just a geographical movement down to the heart of Europe but also to a cultural area that historically had a different model of marriage from Scandinavia; the Spanish case sees the movement extend, geographically and culturally, further south, the first area where Catholicism still exercises significant cultural influence.

All this might seem some distance from Malta's foreseeable future. But that's not quite right. What happens in the rest of Europe has some cultural impact on us. Besides, the truth matters; it is already being debated, however sporadically, in our newspapers. And since there is no doubt that our society still has a lot to learn about the dignity of homosexual persons, we might well ask if there is something to learn from the continental experience, which now, in Scandinavia, stretches back almost a generation.

Unfortunately, the first thing to learn from the various heated national debates that preceded legalisation is that one should treat the claims of both sides with considerable caution, if not outright scepticism.

Pro-gay marriage (or registration) advocates often argue that the move would strengthen marriage in countries, like Denmark and Norway, where marriage is becoming a minority option for couples with children. But this did not happen. Relatively few gay couples have taken up marriage or partnerships where these are possible (probably the main reason why in some countries the rate of homosexual "divorce" is lower than the heterosexual one). The rate of divorce did fall for a while, in a country like Denmark - but the most convincing explanation is that this is because the rate of marriage was also going down: fewer marriages in the first place, less divorce (divorce rates do not register dissolution of cohabiting couples).

But these same statistics cut the other way. If the number of gay couples taking up the marriage option is small, how come they are expected to have such a large direct impact? Especially since gay marriage or partnerships were introduced in countries that were already experiencing a meltdown in the rate of marriage. In fact, the anti-gay marriage lobby has never, to my knowledge, offered any evidence of the supposed direct impact - only statistical correlations which, of course, cannot tell you if the same trend would have happened anyway.

So much was admitted by five Dutch scholars who last year wrote an open letter to all parliaments in the world that were debating same-sex marriage. Three years after Holland legalised same-sex marriage, these scholars underlined the decline of Dutch marriage and a noteworthy rise in births outside marriage. They admitted they had no evidence of one kind of marriage undermining the other. But one factor they did blame was the campaign for gay marriage. Note: not the same-sex legislation in itself but the 10-year pro-gay marriage campaign that had preceded it, which had emphasised that the character of marriage had to do with the couple not with parenting. The result, according to these scholars and others, like the conservative US commentator Stanley Kurtz, is that in Holland as elsewhere there was a cultural institutionalisation of an existing heterosexual tendency to separate parenting from marriage - a separation that makes families more fragile, since cohabiting couples tend to split up at much higher rates than married couples.

The argument now is persuasive. But it becomes so by significantly changing the terms of the debate. First, gay marriage becomes only one factor among several that reinforces the decline of heterosexual marriage and family fragility - it is no longer monumental perversity, as it sometimes is made out to be, and its impact is not direct.

Second, it shifts a considerable part of the burden of responsibility for the outcome. It is the inability of so many modern heterosexual men and women to commit to each other for very long that creates the troubled environment that gives official recognition of the sexuality of a small minority its proportionately large effect.

The burden lies with heterosexual instability, which magnifies the impact.

That is probably not an argument that will win the approval of the gay rights movement. But then that is not why I made it. The argument is about truth: that it matters; and that, with some kind of truth, it helps to look at what actually goes on.