Acting Labor leader Tanya Plibersek 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 will argue Labor should change its party platform at the party's national conference in July, in what is likely to spark a heated internal debate.

Labor's platform currently supports same-sex marriage but does not make it compulsory for Labor MPs to support it in a parliamentary vote.

The acting Labor leader, who holds the inner-city seat of Sydney with high levels of support for the reform, said it was important to make the change to a binding vote because same-sex marriage was an issue of "legal equality".

"Labor has always been a party that is opposed to discrimination," Ms Plibersek told Fairfax Media in an interview last week.

"It is a clear question. Do we support legal discrimination against one group in this country? Or do we not?"

The Labor platform also allows a conscience vote on abortion. Ms Plibersek reasoned that same-sex marriage was not a matter of life and death and should not have the same conditions.

ALP presidential candidate Louise Pratt and other members of Rainbow Labor are also pushing to bind MPs to vote for same-sex marriage.

Ms Plibersek argued the time was right for platform change because there had been "a significant step forward" in terms of community support for same-sex marriage. A Crosby Textor poll last year found that 72 per cent of those surveyed backed same-sex marriage.

At the Labor Party's last national conference in 2011, it voted 208 to 184 in support of a conscience vote.

While Ms Plibersek is optimistic the campaign at the upcoming conference will succeed, she also expects there will be a "few people" who do not agree with the move. The conservative Catholic national president of the largest trade union, the Shop, Distributive and Allied Employees Association, Joe de Bruyn, is likely to be a high profile opponent.

In 2012, when both the lower and upper houses voted down same-sex marriage bills, senior Labor MPs such as Tony Burke, Stephen Conroy, Chris Bowen, Ed Husic, Joel Fitzgibbon and Jacinta Collins voted against the reform for a variety of reasons. The bill was defeated 98 votes to 42, with the Coalition voting against same-sex marriage as a bloc.

A binding vote also raises the prospect that some Labor MPs might refuse to comply. On Sunday, when asked if some Labor MPs would cross the floor if they were bound to support same-sex marriage, Labor's health spokeswoman, Catherine King, told ABC TV: "I think that is understood".

While Ms King said it was an "anomaly" that Labor allowed a conscience vote and confirmed she supported same-sex marriage, she noted: "I am also a bit worried that this idea about not having a conscience vote might distract from the overall issue. It is potentially picking a fight we may not need to have."

But if Labor MPs voted as a bloc it would provide a significant boost to the same-sex marriage campaign inside Parliament House, which has focused on the Coalition since the 2013 election.

Prime Minister Tony Abbott – an opponent of same-sex marriage – has said that the matter of a free vote for Liberal MPs would be one for the post- election party room to determine, but the party room has not discussed it yet. It recently passed up the opportunity to use libertarian senator David Leyonhjelm's same-sex marriage bill to kick off debate.

If Liberal MPs were given a free vote in the current Parliament, same-sex marriage advocates think the reform has a chance of passing the Senate. There is much less confidence about the lower house, where the Coalition has a large majority.

Ms Plibersek has previously said she would introduce a private members bill to legalise same-sex marriage in the current Parliament if she could get a Liberal co-sponsor and the Liberal Party was also allowed a free vote.

But if Labor changed its platform, it could also go to the next election, pledging to reform the marriage act.

"Relying on numbers from a conscience vote is uncertain. There is a certain gravity that attaches to a government bill, as opposed to a private members bill, and I think that sends a strong message too," she said.

Ms Plibersek said that she was not asking any church to marry same-sex couples, arguing marriage was more than just a religious act.

"It's symbolic in our society and gives legal rights and responsibilities."

A long-time supporter of same-sex marriage, Ms Plibersek said her position was "fundamental to my politics".

But she has also been moved by gay friends who are in committed relationships, in some cases for longer than she and her husband, Michael Coutts-Trotter.

"How is it fair that I'm allowed to get married and they can't?"

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