When the Marriage Equality Bill passes its final vote in Parliament, possibly tonight, it will signify a marked social change. Less than a decade ago Helen Clark's Government dared not extend the definition of marriage to same-sex couples, offering them a legal equivalent called civil union. Since then, public opinion has undergone a sea change.
It has happened not only in New Zealand but in Australia, the United States and Europe. Quite suddenly, most people have come around to the view that homosexual commitments deserve equal recognition.
Leadership has helped. When President Obama declared his support for the cause, he probably knew from polling that it would no longer bring public wrath down upon him. In this country, John Key's support removed much of the conservative opposition that the bill sponsored by Labour's Louisa Wall might have met.
Its passage through Parliament has been unusually smooth by comparison with most "conscience issues" that allow MPs a free vote. No member of the House has become identified as a campaigner against it.
Outside Parliament, though, the debate has not been one-sided. Thoughtful contributions to our opinion pages have argued that marriage between a man and a woman is too important to social cohesion for its heterosexual definition to be lost. Marriage, they said, is not simply a declaration of love and commitment, it is the legitimation of procreation and the formation of families.
If its definition is to be detached from that purpose and marriage is to mean any form of human bond, what next, they asked. Might a commitment of more than two people have a right to the same recognition? Polygamy is permitted in some cultures. Why restrict the recognition to sexual relationships? One woman who lived with her sister wrote about their enduring non-sexual life together and wondered whether, in the name of equality, they too should be allowed to marry.
Marriage, as a professor of law pointed out on our pages yesterday, has been instituted in every culture, tribe and race since antiquity as the union of a man and a woman. It has never, until now, included a category of relationships that have no reproductive capacity and cannot provide a child with the care of two biological parents.
In response, proponents of the bill pointed out that not all married men and women can, or want to, have children and that marriage has served many other social purposes, such as protecting property rights and establishing legitimate heirs. They have agreed children should be raised by their mother and father but many are left with a sole parent and they have argued an extra parent of either sex might be a bonus.
As the debate proceeded public opinion became more divided. Support for the bill dropped to 48 per cent in a Herald-DigiPoll survey last month but a slight majority still favoured the change. If the bill was submitted to a referendum it would probably pass, though perhaps not as heavily as it will be favoured in Parliament at the final reading.
Those who worry that something of value will be lost can probably relax. Laws cannot change the ordinary meaning of words such as marriage, bride, groom, husband, wife, mother and father. Marriages for heterosexuals, including the blessings that believers obtain from churches, will not be diminished.
It remains to be seen whether same-sex couples marry in large numbers but their right to do so will be a significant achievement, another legal statement of equality. The gay community's fight for the right to marry pays tribute, in its way, to the inherent value of the institution.
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