Bill Shorten has narrowly avoided a Left faction attempt to immediately bind Labor MPs to support same-sex marriage equality from the next election onwards but the affirmative policy will still become binding on Labor MPs within four years.

And Mr Shorten has pledged that a Shorten Labor government would move to make it law within its first 100 days.

In moving the motion, Mr Shorten pleaded with Prime Minister Tony Abbott to recognise that love between two people was personal and sacred and should not be subject to political partisanship.

"If Mr Abbott fails to give his members of Parliament a free vote this year he has failed the challenge of our democracy, he's politicising the issue of marriage equality when he doesn't need to," he said.

"He should trust his members of Parliament to follow their views, not his particular way of thinking.

"Same-sex couples have waited too long, Australia is trailing the world. The debate about marriage equality has simply gone on too long."

Under a compromise deal nutted out on the sidelines of Labor's 47th national conference, between Mr Shorten, his deputy Tanya Plibersek, and Senate leader, Penny Wong, Labor MPs will be granted a conscience vote when the matter of same-sex marriage comes before Parliament at any time during the remainder of this parliamentary term and for the duration of the following one.

However, after that, a positive vote for the reform will become mandatory for all of ALP parliamentarians, meaning any MPs voting against it on religious or other grounds would face automatic expulsion, consistent with long-standing ALP rules.

Speaking to the motion, an emotional Penny Wong, the nation's highest-profile member of a same-sex couple, thanked her party for its courage and commitment.

"Of course there are many of us, and I'm one, who would have liked [progress] sooner. But delegates, what I say is this: this resolution does what we want, which is to end the conscience vote and you have the alternative prime minster of Australia giving you the commitment of what a Labor government would do: within 100 days, a marriage equality bill.

"There is nothing  to fear from equality. Progress has never been easily won, reform is never easily won."

The outcome represents an immediate-term win for Mr Shorten and another setback for the Left faction which suffered a series of defeats at the weekend, not least in its attempt to prohibit boat turn-backs as part of a future Labor government's suite of border protection policies.

But senior Left figures were not quibbling, with most delighted at the prospect of a binding vote within four years.

Among the other crucial figures reportedly involved in the deal over same-sex marriage policy, was the morally conservative trade union leader Joe Debruyn, of the powerful Shop, Distributive and Allied Employees Association – a key right-wing union.

But while the hybrid voluntary-compulsory formula is a win for Mr Shorten who publicly opposed a plan initially backed by Ms Plibersek to bind Labor MPs immediately, the prospect of mandatory policy by 2019 may also offer Mr Abbott the political cover he needs to deny his own Liberal and Nationals MPs a free vote in this Parliament.

He is likely to argue that he should not be called on to give a free vote to his MPs by an opposition party intending to bind its own.

Mr Abbott has so far refused to grant his MPs that latitude to vote with their individual conscience.

While a growing number of Coalition MPs have indicated a willingness to support marriage equality, Mr Abbott believes the issue is peripheral to the major challenges facing the nation. He has told colleagues he does not believe it needs to come to a vote at all this term – a denial that would mean the matter of a free vote would not arise.

The managing director of the Australian Christian Lobby, Lyle Shelton, said the compromise deal was "a blow to freedom and tolerance".

"If Labor decides to be intolerant with its own parliamentarians, it is sure to be intolerant when in power towards the millions of Australians of ethnic and religious background who will never support a state-imposed redefinition of marriage," Mr Shelton said.

Coalition MP Warren Entsch is one of the co-sponsors of a cross-party bill providing for marriage equality which will be introduced into Parliament when it returns next month.

However, because of Mr Abbott's personal position, it is not clear if it will be afforded the time for debate, and therefore a final vote.

Seconding the motion, Ms Plibersek praised Mr Shorten as a Labor leader who had "nailed his colours to the mast", but slammed Mr Abbott for still blocking "even a discussion" in his own party.

She said it was still possible to achieve marriage equality by Christmas, but said if it didn't happen, Labor would certainly achieve it within the first 100 days of its next government.

The national director of Australian Marriage Equality, Rodney Croome, said advocates would continue to focus on campaigning for Coalition MPs to have a conscience vote.

"It is now cemented in place that marriage equality will be achieved through a cross-party conscience vote so the focus must be on the Coalition allowing a free vote if Australia is to move forward," Mr Croome said.

- with Stephanie Peatling

​Follow us on Twitter