Legalising same-sex marriage could contravene human rights and make it harder for heterosexual couples to adopt children, coalition senators have told parliament.
They were taking part in a debate on a Greens bill that seeks to define marriage as a union of two people and recognise gay couples married overseas.
Nationals senator Matthew Canavan said the evidence was "abundantly clear" that outcomes for children were best on average when their biological parents were married.
Love was not a sufficient condition for marriage because it was also about creating children, something only heterosexual couples could do.
Changing the Marriage Act in the way the bill proposed would not be a celebration of diversity, Senator Canavan said.
"It will be a celebration of uniformity."
Senator Canavan again argued that any change to the Marriage Act that did not respect religious views would be a contravention of human rights.
Government plans to hold a national vote on same-sex marriage after the next election were criticised by Greens and some Labor senators who fear a plebiscite will give voice to bigots.
They also argue it's a waste of money because polling shows a majority of Australians support marriage equality.
"Can anyone seriously suggest that we should be spending more than $100 million of taxpayer funds on a question we already know the answer to?" Greens senator Robert Simms said.
"The last thing I want is to see taxpayer money being spent on a divisive campaign against marriage equality, what in effect would become a state-sanctioned state-funded hate campaign."
Greens senator Janet Rice said it was time to change laws that would force her to divorce her transgender wife of 30 years if she were to update her birth certificate to reflect a change of gender.
Debate on the bill was adjourned.
