Neither prospective Labor leader will back moves to force all Labor MPs to vote in favour of same sex marriage.
The party's platform currently supports gay marriage but gives its parliamentary representatives a free vote on the matter.
But Australian Workers Union boss Paul Howes thinks that's a mistake.
He'll tell an Australian Marriage Equality forum in Sydney on Wednesday it's an issue of social justice, not individual conscience.
The forum will include supporters of same sex marriage across party lines, including Tony Abbott's sister Christine Forster.
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"Just like with racial discrimination, it is vital to understand that our past discrimination against people in same sex relationships is not some sort of fundamental starting point for humanity," Mr Howes will say.
"I want tonight to urge my party to finally disown this phony notion that we should be affording equal respect to both sides of the gay marriage debate, as if it were some exquisitely balanced moral quandary that could never be unlocked by mere mortals. It is not."
He wants the party's 2014 national conference to change the rules so there isn't a conscience vote.
But both leadership candidates Bill Shorten and Anthony Albanese say while they personally support same sex marriage, it should be a conscience matter.
Mr Albanese said legalising same sex marriage was a reform whose time had come and parliament should act.
"But equally, I believe this can best be achieved when all members and senators are given a free vote on any future legislation," he told AAP.
With Labor MPs split on the issue and the coalition insisting its politicians vote against same sex marriage, several bills put to federal parliament have been doomed.
Mr Shorten's spokeswoman said he was of the view that limiting the vote would not be consistent with his vision for a diverse, inclusive Labor Party.
The Australian Christian Lobby managing director Lyle Shelton said the issue had been given a fair go.
"It would be damaging for to Labor continue to tie itself to this agenda," he said.
"Labor's 2014 national conference should really be focussing on how the party can better appeal to the centre on social policy, including how best to promote policy which supports, wherever possible, a mother and father for children."
