Family First national director Bob McCoskrie says the push for same-sex marriage in New Zealand is a stepping-stone to legalising polygamy.
Mr McCoskrie said a new poll by Key Research that showed 53.9 per cent of respondents supported gay marriage was an indicator that fewer people were in favour of marriage reform compared to a year ago.
He said the number had dropped from 63 per cent in a May poll, as people "got past the slogan of "marriage equality' and debated the real impact of redefining marriage''.
"There is absolutely no need to redefine marriage to provide legal recognition and protection for same-sex relationships,'' he said.
If the argument was "about real equality'' then it would mean a push to legalise multiple marriages or marriages between teenagers, Mr McCoskrie said.
"If you're talking about real equality that means it should be available to anybody. That includes three or four who want to get married, that includes teenagers who want to get married, ... married people who want to marry somebody else at the same time.
"It's on the agenda and everybody knows it. It's on the long-term agenda.''
The poll found the majority of younger people gave gay marriage their support, and the majority of those aged 75-plus opposed it.
Females were more supportive of same-sex marriage - more than 60 per cent of women supported legalisation, compared to 47 per cent of men.
Labour MP Louisa Wall, whose Marriage Equality Bill passed its first reading last year by 80 votes to 40, said there was clear widespread support for same-sex marriage.
She said the strongest evidence was in the large percentage of young people who were constantly found by polls to support gay marriage.
Ms Wall said it showed greater equality and a lack of discrimination by younger generations who had grown up seeing homosexuals as normal.
"My primary driver has always been about equality and non-discrimination. I don't believe we are redefining marriage,'' Ms Wall said.
Mr McCoskrie said same-sex marriage should be put to a referendum.
"In 2004, the Government introduced civil unions and changed over 150 pieces of legislation to achieve [legal recognition for same-sex couples],'' he said.
"There is no so-called discrimination.
"The real question is 'why are further special rights now being demanded?'''
Ms Wall's Marriage Equality Bill has been sent to a select committee, which will report to Parliament on February 28.
A second reading is scheduled for March 20 and a third and final reading could happen by May.
Do you think that same-sex civil unions should be extended to marriage?
* Yes 53.9%
* No 38.1%
* Unsure 8%
Poll: Key Research. Commissioned by: Herald on Sunday