"It's just prejudice, and there's no reason for it," said Green MP Metiria Turei, whose Adoption (Equity) Amendment Bill was put into the private member's ballot last week.
The bill would give civil union and de facto couples the same adoption rights as married couples. They would still have to meet the current standard of being a "fit and proper person".
Ms Turei said Labour should have reformed adoption law in the last parliamentary term but backed off because the issue was "too politically sensitive".
Before it can even be considered, Ms Turei's bill will have to be drawn from the ballot. The number of bills in the ballot fluctuates. Last week there were 30.
Most members' bills do not pass. In the last parliamentary term, only eight were made into law.
Justice Minister Mark Burton said through a spokesperson that although he had not yet seen the bill, he was "not unsympathetic" to its purpose and would give it careful consideration if it was drawn.
The bill is likely to spark debate over the suitability of same-sex couples as parents.
Brendan Malone of Catholic group Family Life International said same-sex couples were a "second-rate alternative" to two hetero-sexual parents.
"It's quite clear from the research that children in loving heterosexual marriages do better than other parenting arrangements," he said.
"It seems crazy that we would promote something in law that is of no benefit to children and has been shown by research to be detrimental to them."
In 2000, the Law Commission recommended lifting the prohibitions on adoption by de facto and same-sex couples. Its report said there was not enough evidence to show that being adopted by a same-sex couple was against a child's best interests.
Gay Auckland Business Association president Richard James, who lives with his partner and has a young daughter from a previous relationship, said sexual orientation was irrelevant when it came to good parenting.
"So long as you are giving children a balanced lifestyle, then it shouldn't make any difference."
Professor Mark Henaghan, dean of law at Otago University, said the Adoption Act, which was passed in 1955, had stayed "frozen in time" and should be updated.
"In 1955 it wasn't even thought that there would be de facto couples, let alone same-sex," he said.
The Adoption Act was the last piece of family law legislation not to recognise same-sex couples.
Adoption rates have plummeted in recent years, from 3600 adoptions in 1972 to about 300 a year now.