The federal Labor MP leading the opposition’s marriage equality campaign has vowed not to boycott a postal plebiscite on legalising same-sex marriage.

The government will resubmit its plebiscite plan to parliament this week for a vote, reportedly on November 25.

If the bill fails a second time – which appears certain unless key crossbenchers flip their positions – a voluntary postal ballot will be conducted.

But Opposition frontbencher Terri Butler says she is concerned some Australians won’t participate in a popular non-binding vote because of its “funny” mechanism.

“If it goes ahead, it’s going to lack legitimacy,” she told Sky News on Tuesday.

But the MP is “certainly not going to boycott” a poll.
 Junior minister Michael Sukkar, an opponent of same-sex marriage, said all coalition MPs should abide by the result of any plebiscite or postal ballot.

In a move likely to anger Liberal MPs who support gay marriage, cabinet minister Mathias Cormann says the government will not allow a private bill to be put to parliament if a majority of Australians voted no.

But if a majority of Australians voted ‘yes’ the government would facilitate private legislation to change the Marriage Act before the end of the year.

“If the outcome is no, then the government will not do that,” Senator Cormann said.

That could force the hand of seven Liberal MPs who supported a free vote on the issue during a partyroom debate on Monday.

Backbencher Warren Entsch is reserving his right to cross the floor of parliament to vote against the government.

The debate comes as an Essential poll found 43 per cent of voters approved holding a voluntary postal plebiscite followed by a vote in parliament, while 38 per cent disapproved.

Labor leader Bill Shorten said right-wing Liberals were dictating policy to the prime minister.

“We should just let people who love each other marry,” he told reporters.

Advocacy group Australian Marriage Equality has legal advice it says confirms a postal vote would be unconstitutional and warned of a potential High Court challenge.

Co-chair Alex Greenwich says the government’s approach to marriage equality has gone “well beyond a joke”.

But the Australian Christian Lobby accused the seven Liberal MPs of holding the country to ransom over the issue.

“They have hijacked all of our national politics at a time when the prime minister is trying to deal with terrorism,” managing director Lyle Shelton told ABC TV.