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?”