Greens MP Cate Faehrmann introduced a private members' motion last week for the Commonwealth Marriage Act to be amended, with MPs from both major parties split on the issue. The Legislative Council voted 22 to 16 in favour of the motion. Coalition MPs, including National Trevor Kahn, previously expressed support for move that calls on the Federal Government to allow gay marriage.
But Liberal Matthew Mason-Cox today said same-sex marriage could lead to polygamy.
"Indeed, if one was to take the notion of equality of marriage to its logical conclusion, then there would be no reason to stand in the way of polygamist marriages, or other variants," he told the chamber today.
"This is the so-called slippery slope in this debate which has manifested itself overseas in some jurisdictions where same-sex marriage has been allowed."
Labor MP Helen Westwood, who is in a lesbian relationship, spoke in favour of the motion.
"This is a human rights issue and I believe very strongly the laws that discriminate against one group of Australians should amended," she said.
Christian Democrats MP Fred Nile, who strongly opposes gay marriage, has previously spoken against the motion, and moved an amendment noting how the European Court of Human Rights did not consider gay marriage to be an inherent right.
Premier Barry O'Farrell refused to confirm whether he backed the motion.
"It's an issue for the upper house, should it come to the lower house I'll make my views clear then," he said in Sydney.
The Premier has allowed Coalition MPs a conscience vote on the issue, unlike Federal Opposition Leader Tony Abbott.
Mr O'Farrell dismissed suggestions the motion, if passed, may put pressure on Mr Abbott to allow a conscience vote when the Bill comes before federal parliament.
"I think Tony Abbott is the person least affected by pressure that I know," he said.