One of the Government’s claims in favour of gay marriage is that it will strengthen the institution of marriage. This idea has been particularly embraced by Conservative Party exponents ever since the prime minister announced, last summer, out of the blue, that his Government would legislate for SSM because he believed in “the ties that bind”.

Like the justice case for SSM —  that it will ‘increase equality’ — the conservative case is  intuitively simple: more people marrying will mean greater social glue. Some Tory advocates, such as the MP for Henley, John Howell, are a little less sure. At the end of a paper he has sent to his constituents (download John Howell MP paper on gay marriage) he says allowing gay marriage “is not a fundamental change at all and will, on the basis of evidence from elsewhere around the world, have no effect on the structure of society”.

Having no effect on society is rather less than the PM claims. But is even this true?

He backs the assertion with reference to one case, that of Spain, where since SSM was introduced in 2005 “few will ever have seen [a gay wedding] take place since they account for less than 2% of all marriages in the country. As to the effect of gay marriage on traditional marriage; there does not appear to have been any negative effect.”

There is no footnote or further evidence for this statement, which contradicts the available statistics showing that since 2005 the rate of decline in numbers marrying in Spain has been twice that of France and Italy. Of course, there may be other reasons for that decline — the economic crisis, for example, or the introduction of quickie divorces at the same time as the gay marriage law, which left many Spaniards wondering what the point of marriage was. But nor do the statistics allow Howell to claim that the effect of SSM has been neutral.

Opponents of SSM say it weakens the law’s understanding of marriage. A comprehensive, conjugal institution — the sexual union of man and woman, apt for procreation, and allowing children to be raised by their birth-parents in an environment of sexual exclusivity, commitment and stability — is replaced by a domestic partnership in which sex, fidelity, and children are removed or optional. It seems obvious that, having removed one defining element of marriage — in this case sexual difference — the rational basis for the other characteristics is weakened in the public mind.

Take Holland, where after allowing same-sex unions the law now it possible for three-way relationships to be recognised as “civil unions”. In the Brazilian state of Sao Paulo, the law does not allow it; but a notary who married a threesome, arguing that since introducing SSM in that state the law did not prevent it, had logic on her side. In Canada, meanwhile, a British Columbia court considered whether to introduce polygamy following an application by fundamentalist Mormons to have their union recognised. The Court rejected the idea, saying that the idea of two parties to a marriage was a core element of western tradition. Amazingly — because by then SSM was legal in Canada — the Court did not see the contradiction involved. But others do. Writes Bradley Miller:

The lesson is this: a society that institutionalizes same-sex marriage needn’t necessarily institutionalize polygamy. But the example from British Columbia suggests that the only way to do so is to ignore principle. The polygamy case’s reasoning gave no convincing explanation why it would be discriminatory not to extend the marriage franchise to gays and lesbians, but not discriminatory to draw the line at polygamists and polyamorists. In fact, the judgment looks like it rests on animus toward polygamists and polyamorists, which is not a stable juridical foundation.

In Mexico City, which legalised SSM in 2009, there are plans to introduce temporary marriage licences lasting just two years. After all, if marriage can be changed in one particular (gender complementarity), why not another — the idea of marriage for life?

The point is not that same-sex marriage leads inexorably to the recognition of polygamy and polyamory and temporary unions. The point is that it sets up confusion in the public mind about the nature and significance of marriage. Law teaches — or fails to teach – -culture. And the lessons of SSM all point to a downgrading of the public understanding of marriage.

The other huge problem with the prime minister’s case for SSM strengthening marriage is that gay people themselves are fairly indifferent to it. We already know from polling in the UK that the take-up will be low; and the experience of other countries demonstrates it. A 2005 Netherlands report indicated just 12 per cent of same-sex couples had married (as opposed to 82% of heterosexuals). In Canada, there are just 21,000 married same-sex couples out of 6.29 total married couples. And according to the Spanish National Institute of Statistics, the number of same-sex unions under the 2005 Act were: 1,275 (0,6% of the total of marriages) in 2005; 4,574(2,16% of the total of marriages) in 2006; 3,250 (1,6% of the total of marriages) in 2007, and 3,549 in 2008.

Such tiny numbers indicate a very low probability of marriage being strengthened. But do same-sex marriages give a particularly fine example of longevity and commitment? Here, too, the evidence points in the other direction.

In Netherlands and Belgium the divorce rate among same-sex couples is far higher than among couples as a whole. Belgium is particularly striking: according to this analysis (download mariage-et-divorce-homosexuel-lexperience-des-autres1) male same-sex couples are 21% more likely to divorce than heterosexual couples — while lesbian married couples are an amazing 76% more likely to divorce.

The experience of same-sex marriage is new, and the statistics not always clear or easy to find. But what there is out there is pretty conclusive. Same-sex marriage does not strengthen marriage, but fatally weakens it in the eyes of society.