Conservative Party leader Colin Craig says homosexuality is a choice and society is right to discriminate against gay relationships.
Mr Craig, a Christian, backed church groups who called at a select committee hearing in Auckland yesterday for a royal commission and a referendum on any change to the definition of marriage.
He said the debate about Manurewa MP Louisa Wall's bill to define marriage as the union of any two people regardless of gender was about the value the country placed on its history and traditions.
"It asks whether the history and tradition of marriage as an institution uniting a man and a woman for the benefit of children and society deserves our protection," he said.
"Changes like this should not be made lightly. I am not convinced that there is a compelling reason for change.
"Yes, we are discriminating between relationships. We are saying that marriage between a man and a woman is recognised. We are saying that a relationship between a man and a man, for example, goes down the path of a civil union."
Labour MP Moana Mackey asked Mr Craig if he still believed, as he said last August, that homosexuality was "a choice".
"I do," he said. "It's a choice influenced by a number of things including genetics."
But he declined to be drawn into another question from gay Green MP Kevin Hague about whether he agreed with victim advocate Garth McVicar that gay marriages would lead to more crime. "That is not a link that I draw," he said.
Earlier, Family First director Bob McCoskrie warned that terms such as mother, father, husband and wife could disappear from the law if gay marriage was legalised.
He said Spanish law had recently replaced the terms mother and father with Progenitor A and Progenitor B.
"The US State of Washington is to remove the terms husband and wife from divorce courts. In France the words mother and father will be stripped from official documents."
Tongan churches said the bill would destroy their traditional social structure, which was based on relationships to a person's father and mother.
"Within a marriage between a male and a female, the female person's maternal uncle or 'fa'e tangata' has a different obligation to that of the male person's paternal aunty or 'fahu'," said the Tongan branch of the Methodist Church.
"The fahu, the highest-ranking woman in the family, can only have that rank if her brother is married to a woman and they have children."
Kathleen Tuaitaufoou, a New Zealand-born mother of three, said she wanted her children to grow up in traditional culture.
"We have fa'afine. We have a gay community that is openly homosexual," she said.
"But our gay community said, 'We are not going to be out there saying [gay marriage should be legalised], we are going to say we have our boundaries, we know that marriage belongs to the religious part of our culture'."
The bill's supporters told MPs legalising gay marriage would lift the wellbeing of all gay people by reducing the feeling of being marginalised.
Aids Foundation government relations manager Dr Jason Myers said research showed that gay men in countries that banned gay marriage knew less about HIV, were less likely to be tested for it, were less able to refuse unwanted sex and were more likely to have unprotected sex.
"The internalisation of homophobia leads to a devaluation of self." The hearings continue today at the Holiday Inn at Auckland Airport.