Argument that same-sex marriage will lead to multiple partners is dishonest, says Labour's Louisa Wall.
The MP behind a law change to legalise gay marriage has slammed the "dishonest" argument by opponents that her bill will pave the way to polygamous relationships.
Labour Party MP Louisa Wall said she was frustrated by the more extreme arguments against her bill, which had prompted her to release research showing that no country had legalised polygamous relationships after legalising gay marriage.
Ms Wall said the argument that allowing same-sex marriage would be a stepping stone to multiple partners was undermining an otherwise civilised and principled debate.
"Everyone has built an opposition based on a belief or a value ... but for some to purposefully mislead not only the public, but to also scaremonger, is fundamentally dishonest," she said.
The concern about creating a "slippery slope" to polygamy was raised at the first reading of the bill by National MP for Wairarapa John Hayes, and has been echoed by submitters to the select committee considering the legislation, in particular the lobby group Family First.
Family First founder Bob McCoskrie said he believed legalising polygamy was "on the long-term agenda".
"If you say that any adults who love each other should not be discriminated against then why limit it to sexuality, why not numbers?"
Ms Wall provided research to the Herald which showed that all of the 11 countries that have legalised gay marriage have outlawed polygamy.
None of the 50 countries that recognised polygamy under civil law formally recognised same-sex relationships.
Ms Wall said that in most cases, polygamy was legal in countries that repressed women, not socially progressive countries like New Zealand.
"You have countries where you can be whipped, fined, flogged, sent to jail for the rest of your life [for being in a gay relationship] so to say that marriage equality is a stepping stone to polygamy completely misrepresents the truth globally."
Asked to respond to Ms Wall's research, Mr McCoskrie said he acknowledged that no countries had legalised same-sex marriage then polygamy, but he felt it was "just a matter of time".
He pointed to government reports in Canada - where gay marriage was legal - which recommended the decriminalisation of polygamy, partly to attract skilled immigrants from countries which allowed multiple partners.
Ms Wall has been highly active in her attempt to quell concerns about her private member's bill.
The MP held forums with Pacific Island churches in South Auckland after some ministers believed they would be forced to marry gay couples if the bill passed.
The select committee considering the bill was likely to make an amendment to make it explicit that churches would retain their freedom of religion and expression.
Countries where same-sex marriage is legal:
Argentina - since 2010
Belgium - 2003
Canada - 2005
Denmark - 2012
Iceland - 2010
Netherlands - 2001
Norway - 2009
Portugal - 2010
South Africa - 2006
Spain - 2005
Sweden - 2009.