Defence Industry Minister Christopher Pyne indicated he would oppose all amendments to senator Dean Smith's bill, deriding the various changes proposed by his Liberal colleagues as unnecessary and superfluous.

Mr Pyne's declaration means there are now enough MPs opposed to the amendments to form a blocking majority of 74 votes, including Labor's 69 MPs, Adam Bandt, Rebekha Sharkie, Andrew Wilkie and Liberal backbencher Warren Entsch.

Fellow Liberal MP Trent Zimmerman took that number to 75, telling Fairfax Media on Tuesday: "I have not seen any amendments that would dissuade me from believing that the bill adopted by the Senate should stand."

Mr Pyne - a long-standing supporter of same-sex marriage - told the chamber he was satisfied the bill already protected religious freedom.

"I do not support the insertion of unnecessary amendments," he said. "Acts of Parliament should not contain superfluous clauses - especially superfluous clauses based on the opinion that Australia's laws don't adequately protect the religious freedoms that we have cherished since Federation. I firmly believe that they do."

In a passionate and personal speech reflecting on his own Christian faith, Mr Pyne said the right for same-sex couples to marry had been "denied too long, too cruelly - and too often with such meagre and patently disingenuous defences".

The denial of civil marriage equality was "a betrayal of the separation of church and state", "hypocritical" and "plainly wrong", he said.

Mr Pyne's opposition to the amendments will give the green light for other MPs to vote against them in what is a conscience vote for all Coalition MPs. Financial Services Minister Kelly O'Dwyer, also a member of the moderate Liberal faction, stated last week that she is unpersuaded by the amendments put forward to date. Education Minister Simon Birmingham and Defence Minister Marise Payne, also moderates, rejected the amendments in the Senate last week.

Their stance contradicts that of Mr Turnbull, who will support moves to allow civil celebrants to discriminate against gay couples even though they are doomed to fail. On Tuesday, the PM said it was his ministers' "absolute right" to disagree with him and vote as they see fit.

"I am not sure what part of free [vote] you do not understand," he said.

But at a Coalition party room meeting on Tuesday, Mr Turnbull and the man he replaced, Tony Abbott, clashed over Mr Abbott's proposal to add a so-called "pious amendment" to the bill. The motion, which would not alter the bill, is a declaration that nobody should "suffer any adverse effects" from their beliefs about marriage.

Former prime minister Tony Abbott and Christopher Pyne clashed over Mr Abbott's proposed 'pious amendment'. Photo: Alex Ellinghausen

Several MPs in the room told Fairfax Media the exchange was "terse" or "heated". A frustrated Mr Turnbull eventually told Mr Abbott he would not support the motion. "He basically said to Abbott, 'I have been married to Lucy for 38 years and I know the importance of marriage'," one MP recounted.

"It's the strongest I've seen Malcolm. He really put Abbott in his box," another MP said.

Mr Pyne then intervened and said the pious amendment would stop the bill in its tracks, that debate would have to be restarted and that it would cause a major problem. Mr Zimmerman echoed Mr Pyne's intervention, while Kevin Andrews backed Mr Abbott.

Alex Greenwich, co-chair of the Equality Campaign, said he was "grateful to all the MPs across the Parliament who are voting down amendments, and in doing so ensuring the will of the people and the government's promise are fulfilled to achieve marriage equality this week".