The ALP's national conference decision to give its MPs a free vote on same-sex marriage for the remainder of this parliamentary term and the next has been seized on by conservatives in the Liberal party room who believe it provides Tony Abbott with a justification to do nothing.

Right-wing Liberals say Labor's "ridiculous" policy of allowing its MPs a conscience vote on same-sex marriage equality only until 2019, and binding them to support it thereafter, has given Mr Abbott all the cover he needs to deny freeing up Liberals.

"Why would we bind ours," said one MP, who declined to be named.

Liberals at present are functionally obliged to oppose any change to the Marriage Act, which was amended by the Howard government to specifically define marriage as between a man and a woman to the exclusion of all others.

Technically, however, only frontbenchers would be in breach of the rules if they broke ranks to support a reform bill in Parliament. Backbenchers can vote as they wish in such a circumstance, but in the absence of a specific permission to vote freely, many would worry about career advancement if they opposed the Prime Minister's settled policy.

While Mr Abbott promised before the 2013 election that the question of a conscience vote or otherwise would be decided by the party room once in office, he has since shown no inclination to bring the matter to a head.

That is despite a private member's bill coming forward within weeks sponsored by two senior Liberals, Warren Entsch and Teresa Gambaro. It is not clear that the bill will be afforded time for a debate and then a final vote, and it is also not clear how it would fare if it were put to a vote, with most Labor MPs to back it but by no means all.

Advocates of reform - including cabinet ministers who are bound to support the leader unless granted the free vote - want the Entsch-Gambaro bill brought on.

But opponents say that bill should be treated like any other private member's bill, which is code for saying it should never come to a vote.

The influential Social Services Minister Scott Morrison slammed Labor's hybrid position on Monday, in a sign of the arguments to come.

"You either have a conscience vote or you don't," he told Fairfax Radio.

"The fact that you can just trade away a member's conscience and that that be done by a union-driven conference as opposed to members themselves making this decision I think says everything you need to know about the Labor Party. And for them to now be lecturing others about conscience votes I think makes a complete mockery of the whole argument that Bill Shorten is trying to make."

Another Liberal said Mr Abbott would resist the change for as long as he could, predicting there was no need for it to come up before the election, even if that is a year away.

However, others said Labor's position had not changed anything on the Liberal side.

Opinion polls show public support for equality is building, accelerated by the stunning 62 per cent "yes" vote in a referendum on the question in conservative and predominantly Catholic Ireland in May.

Other countries are also moving, with the US Supreme Court recently identifying same-sex marriage equality as a constitutional right in all US states.

At Labor's conference, Mr Shorten argued strongly for a continuation of a free vote, aware that some Catholic and other religious MPs feel they could never support the change. But a compromise with the resurgent Left saw a half-half policy agreed on.

The ALP's Left faction had wanted an immediate binding policy, arguing the change was not a matter of conscience, but a fundamental question of equality.

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