Opponents of Labour MP Louisa Wall's Marriage Amendment Bill say it could open the way for polygamy, incestuous relationships and breaches of the Human Rights Act.
Anti-gay marriage lobbyists and religious groups are making a last minute appeal to Government to not change the definition of marriage.
The first reading of Labour MP Louisa Wall's Marriage Amendment Act bill that would allow same-sex people to marry will be heard in Parliament tomorrow.
Lobby group Family First has presented a petition signed by 50,000 people to MPs from Labour, National and NZ First, calling on them to reject the bill.
The petition said: "I support the definition of marriage in New Zealand being maintained as one man one woman. I oppose any attempt to redefine it."
The protesters, led by conservative lobby group Family First, wanted marriage to remain between a man and a woman.
The group was met on Parliament steps by National MP Alfred Ngaro, Labour MPs Su'a William Sio and Ross Robertson, who opposed the bill, and New Zealand First MP Asenati Lole-Taylor, who wanted a referendum on the issue.
Family First's national director, Bob McCoskrie said it was an incredible response considering the campaign was only launched a month ago.
"It shows the level of public opposition to the bill, and the signatures continue to pour in," he said.
He said same-sex couples already had legal recognition through civil unions, so there was no need to redefine marriage.
"It's not discriminatory to support the existing definition of marriage. Many homosexuals do not support the call for same-sex marriage - yet they are certainly not 'homophobic' or 'bigoted' which are lazy labels given to anyone supporting the current definition of marriage."
If marriage was redefined once, there was nothing to stop it being redefined again to allow polygamy, polyamory and adult incest relationships, he said.
"Throughout history and in virtually all human societies, marriage has always been the union of a man and a woman. Marriage reflects the complementary natures of men and women."
New Zealand Christian Network has pushed for a Royal Commission to investigate the issues involved in the bill.
The network's national director Glyn Carpenter said the bill was significant.
"It is a bill with potentially enormous sociological and economic implications for New Zealand society.
"In nearly every area the Government requires a strong evidential base before proceeding with policy changes which might have such a major impact on society. This bill is of such significance that a Royal Commission is needed to investigate it properly rather than the usual select committee process or a referendum."
Mr Carpenter said the bill also involved the redefinition of a word that had a specific and clear meaning, which achieved little other than creating confusion around the word and the importance to society of the institution it describes.
"If such a definition is changed, then it will require the invention of another word to describe the unique meaning which the original word had."
If politicians took the step of treating marriage as something which could be redefined at whim rather than recognising its natural-based dimension, then there was no rational argument which could be used to oppose any other variation which a minority group may push for in the future, he said.
A legal opinion by lawyer Ian Bassett said a Human Rights Commission statement that religious officials and leaders were free to refuse to perform marriages that were not in accordance with their religious beliefs, was incorrect.
He said if celebrants and church leaders refused to perform marriages, or hoteliers and others supplying services to the public refused to supply services to same-sex couples for a wedding, they would be breaching the Human Rights Act.