Senator Penny Wong and Senator Cory Bernardi will debate same-sex marriage on July 29. Photo: Alex Ellinghausen

They're two of the Parliament's fiercest voices on same-sex marriage.

One is a passionate advocate of marriage equality, the other a staunch defender of the status quo.

Now Labor Senator Penny Wong and fellow South Australian Senator Cory Bernardi will tackle marriage reform in a debate at the National Press Club.

Penny Wong and partner Sophie Allouache and their daughter Alexandra in 2011. Their second daughter, Hannah, was born earlier this year. Photo: David Mariuz

The pair will face-off at the end of this month before the issue is brought to a head in August with the return of Parliament and the moving of a multi-party marriage equality bill.

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For the bill to have any chance of success, Prime Minister Tony Abbott must allow his MPs to have a free vote.

Senator Bernardi is among conservative Coalition MPs rallying against any change to the Marriage Act and has criticised frontbench colleagues who have spoken out in favour of same-sex marriage.

He once warned same-sex marriage could lead to acceptance of bestiality, and has since said it could lead to commercialised surrogacy for gay couples.

Senator Bernardi, along with his colleagues Eric Abetz and Barnaby Joyce, copped a blunt message from Senator Wong, who used an interview with the ABC this week to describe recent arguments against same-sex marriage as "inventive".

Senator Wong, who rarely discusses her private life with partner Sophie Allouache and their two daughters, took aim at Coalition MPs who have argued against same-sex marriage on the basis that children should be raised in a marriage between a man and a woman.

"Memo to Eric: we've already got children, all you are doing is saying the parents can't be married," she said. 

Senator Abetz, the government's Senate leader, has also argued Australia should not legalise same-sex marriage before Asian countries enact marriage equality laws and Mr Joyce said this week that reform in Australia would be viewed by Asia as "decadent".

"The thing I think is, people who oppose marriage equality, they are fundamentally saying to Australians 'Look I just think we should discriminate against gay people, that's my view'," Senator Wong said.

"But because that is no longer a powerful argument in the community we see a whole range of new excuses developed.

"This creative one about how Asia will see this, over the weekend; the slippery slope one around polyamory and prior to that, Cory Bernardi suggested bestiality.

"These are people in search of an argument because they know that the heart of their argument is no longer accepted by most of the Australian community."

The debate will take place on July 29.

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