Gay Australians will never be allowed to marry under a Conservative government, says the Labor opposition.
Labor said PM Tony Abbott’s offer of a same-sex marriage plebiscite next year was nothing but a “delaying tactic”.
The attack comes as Mr Abbott faces criticism within his own ranks about how he has handled the issue.
On Tuesday, he blocked a free vote for government MPs on any same-sex marriage bills but said the issue should be “put to the people” after the next election.
Labor leader Bill Shorten called on all Coalition MPs in favour of same-sex marriage to keep lobbying the prime minister.
“The choice in this country is you either have Mr Abbott or you have marriage equality but you can’t have both,” he told reporters on Wednesday.
Mr Shorten has pledged to introduce same-sex marriage legislation within the first 100 days if Labor wins the next election, due in 2016.
Mr Abbott, who personally opposes same-sex marriage, on Wednesday said under Liberal Party policy, any government frontbenchers who defied the agreed position to oppose gay marriage would be sacked.
Backbench coalition MP Warren Entsch plans to introduce a cross-party bill next week to legalise same-sex marriage
Mr Entsch, who has campaigned publicly for marriage equality, told local media he believed some of his colleagues would cross the floor to vote in favour of his bill.
“Absolutely I will be crossing the floor,” he said, “but even with some support I don’t think the support is there to see it succeed”.
Analysis: Jon Donnison, BBC News, Sydney
The gay marriage issue in Australia is an interesting one, not least because Tony Abbott, and indeed politicians in general, are out of touch with public opinion.
Polls show the majority of Australians support same-sex marriage. And that’s why it’s a potentially dangerous issue for Mr Abbott who admits to feeling “a bit threatened” by homosexuality – even though his own sister Christine is a lesbian.
And it’s not just the public Tony Abbott is out of sync with.
Some MPs within his own Liberal Party are in favour of legalising gay marriage.
So if the prime minister continues to deny his parliamentary colleagues a free conscience vote on the issue, he risks alienating some within his own party.
Communications Minister Malcolm Turnbull, who supports same-sex marriage and a free vote on it by government MPs, said the issue would distract from debate about the economy and jobs ahead of next year’s election.
“The attraction of a free vote would have been that the matter would have been settled one way or another in this parliament,” Mr Turnbull told reporters on Wednesday.
He said Mr Abbott’s offer to hold a plebiscite meant same-sex marriage was now “a live issue all the way up to the next election”.
Mr Abbott’s sister Christine Forster, a Liberal local councillor in Sydney, agreed with this assessment.
Ms Forster, who is a lesbian and has publically supported same-sex marriage, told the BBC that she found her brother’s decision “personally very disappointing” and hoped that the party would review its position soon.
But she added: “We both respect each other’s views… and that reflects Australian society as a whole, there are a range of views on this issue.”
The Labor Party has also faced internal debate about the matter.
At a party conference in July, Mr Shorten convinced his colleagues not to adopt a binding vote on same-sex marriage.
Labor deputy Tanya Plibersek had unsuccessfully lobbied her colleagues to amend the Labor platform so that MPs would be forced to vote in favour of same-sex marriage in parliament.