Prime Minister Malcolm Turnbull has pleaded with both sides of the same-sex marriage debate to campaign respectfully, describing the use of abusive language as hurtful and deplorable.

An anti-LGBTI poster featuring the slogan "stop the fags" has reportedly been seen in a Melbourne laneway.

As Australians prepare to vote in a postal survey, Mr Turnbull acknowledged people were likely to say things that are "hurtful, unfair and sometimes cruel".

But rather than stifle free speech, Mr Turnbull said Australians should stand up for any friends and loved ones feeling distressed "at this challenging time".

"This is a time to put your arms around them, to give them your love and support," he told 2DayFM.

The Prime Minister again said he would vote yes in the survey and declared a bill to legalise same-sex marriage would "sail through Parliament" if most Australians backed the change.

He said Australians should not get distracted by "a handful of extreme posters and flyers" and should avoid "caricaturing" those on either side of the debate.

"You cannot ask for respect from the No case if you're not prepared to give respect to the No case," he said.

"The vast majority of people who do not agree with same-sex marriage are not homophobic and do not denigrate gay people."

He also had a message for colleagues who have warned the change could have "wider ramifications", saying the biggest threats to traditional marriage were "desertion, cruelty, neglect, abandonment and indifference".

Former prime minister Tony Abbott has suggested both freedom of religion and freedom of speech would be at risk if same-sex marriage is legalised, and told Australians to vote No if they do not like the direction in which the country is going.

"This isn't just about marriage," he told Sydney's 2GB on Monday.

"There are lots and lots of implications here and we've got to think them through before we take this big leap into what I think is the dark."

While Labor is deeply uncomfortable with the idea of a postal survey, it has promised to campaign for the Yes vote, along with the Greens, Australian Marriage Equality and GetUp.