Stephen Rainbow (on this page yesterday) seems to make a plausible argument that allowing same-sex marriage will attract back to NZ same-sex couples who want to be married - until we look at the international evidence.
In Australia, studies have found that only about one-fifth of homosexuals and lesbians have shown an interest in same-sex marriage. In the Netherlands, the first country to legally recognise same-sex marriage, studies have shown only around 6 per cent of homosexuals have married during the first five years of legalisation.
In Belgium, it is estimated at less than 5 per cent and in Canada, around 10 per cent on average across the provinces. The US state of Massachusetts seems to be doing the strongest with 16.7 per cent.
Supporters of same-sex marriage argue that civil unions are a second-class marriage but there are many same-sex advocates who argue against "marriage" for same-sex couples, and even suggest that the claim is hurtful to those who have deliberately chosen civil unions.
Marriages are a matter of significant public concern, as the record of almost every culture shows. If it weren't for the fact that sexual intercourse between a man and a woman can lead to children and bring with it a further obligation to care for those children, the notion of marriage would probably never have existed, and the state would not have been interested in it.