Union heavyweight and prominent Labor conservative Joe de Bruyn has dismissed Tanya Plibersek's push for a binding vote on the issue, arguing she is just "playing up" to her cosmopolitan inner-city electorate.
Mr de Bruyn, who is the national president of the largest trade union, the Shop, Distributive and Allied Employees Association, also said Ms Plibersek was taking advantage of the fact that Labor leader Bill Shorten is overseas for Anzac Day.
"While the cat's away the mice will play," he said.
Fairfax Media reported on Monday that Ms Plibersek, who is acting Labor leader, is calling on her party to compel MPs to vote for same-sex marriage, ending it as a conscience issue for the ALP.
Ms Plibersek, who holds the inner-city seat of Sydney, will argue Labor should change its party platform at the party's national conference in July,
Labor's platform currently supports same-sex marriage but does not make it compulsory for Labor MPs to support it in a parliamentary vote. Former leaders Julia Gillard and Kevin Rudd voted against same-sex marriage when it was put to a vote in 2012.
Mr de Bruyn said that he did not think the party would vote to change its platform.
"I just don't think it will happen, because I think good sense will prevail," he said.
He added that every ALP member knew that same-sex marriage was an issue "on which there are deeply held views".
"Why would you create this sort of conflict when you don't need to?"
Mr de Bruyn, who as a conservative Catholic is opposed to same-sex marriage, said that there would be "many" Labor MPs in both houses who would cross the floor if they were forced to support same-sex marriage in Parliament.
He said a binding vote would cause internal grief and this was "one reason why the party knows it would be quite foolish if it was to take away the conscience vote".
While Labor's national platform currently only has a conscience vote for same-sex marriage and abortion, he explained that the ALP had a long history of granting conscience votes on a wide-range of issues, from no-fault divorce to gambling laws and the position of the new Parliament House in the 1980s.
"The practice of the party has been a wide and generous granting of the conscience vote. This is where the issue of gay marriage fits it."
The Coalition has a binding vote opposing same-sex marriage, although Tony Abbott indicated before the last election that the Liberal Party would hold a party room discussion on the possibility of giving its MPs a conscience vote.
Mr de Bruyn said that Ms Plibersek was "simply playing to her constituency" in calling for the binding vote.
"What you see here is just politicians positioning for their own advantage."
Ms Plibersek has argued that she is backing the reform to remove "legal discrimination" against gay Australians.
"Labor has always been a party that is opposed to discrimination. It is a clear question. Do we support legal discrimination against one group in this country? Or do we not?"
A profound decision
Liberal Democrat Senator David Leyonhjelm, who has introduced a private member's bill to legalise same-sex marriage, rejected Ms Plibersek's push for a binding vote.
"I think there should be a conscience vote for all politicians," Senator Leyonhjelm said.
"Nobody should be forced to vote against their values on this. I want politicians to reflect the views of the community and the community thinks the time has come for same-sex marriage."
Senator Leyonhjelm, who has been lobbying Coalition MPs to back a conscience vote, said he did not agree with Ms Plibersek's argument that same-sex marriage is an issue of legal discrimination not conscience.
"For some people, their views on marriage are as significant as their views on euthanasia or abortion," he said.
"Getting married is a pretty profound decision."
On Monday Mr Shorten's office pointed Fairfax Media to the Labor leader's previous comments on same-sex marriage.
Mr Shorten supports same-sex marriage as well as a conscience vote.
Same-sex marriage advocates welcomed Ms Plibersek's commitment, describing it as "heartening".
"We welcome Ms Plibersek's support because, if successful, it will help break the current deadlock over marriage equality and bring Labor's approach to marriage equality into line with its other policies," Australian Marriage Equality national director Rodney Croome said.
"As community support for marriage equality continues to grow, it's heartening that political leaders like Ms Plibersek are forging a path forward."
