Both federal leaders have thrown their support behind the "Yes" campaign, days ahead of ballots for the postal survey on same-sex marriage being delivered to more than 16 million Australian voters.
Despite vowing not to campaign, the prime minister launched 'New South Wales Liberals and Nationals for Yes' in Sydney alongside NSW premier Gladys Berejiklian and former NSW Liberal leaders, Kerry Chikarovski, John Brogden, Barry O'Farrell and Nick Greiner on Sunday.
Turnbull quoted former UK prime minister David Cameron saying he doesn't support gay marriage despite being a Conservative, he supports gay marriage because he is a Conservative.
He told the launch there was no evidence that gay couples pose any threat to marriage.
"I have to say I am utterly unpersuaded by the proposition that my marriage to Lucy, 38-years long next March, or indeed any marriage, is undermined by two gay men or two gay women setting up house down the road whether it is called a marriage or not," Turnbull said.
"Let's be honest with each other, let's be really, really honest with each other, the threat to marriage is not gay couples it is a lack of love and commitment."
"Whether it is found in the form of neglect, indifferent, cruelty or adultery to name just a few manifestations of the loveless desert in which too many marriages come to grief."
"If the threat of marriage today is lack of commitment, then surely other couples making and maintaining a commitment sets a good rather than a bad example."
Turnbull said Australians unsure of how to vote in the postal survey should look to other countries where same-sex marriage has been legalised.
“In any one of those nations has the sky fallen in, has life as we know it ground to a halt, has traditional marriage been undermined? And the answer is plainly no,” he said.
He used the launch to once again call for a respectful debate from both sides of the campaign.
Opposition leader Bill Shorten meanwhile, told thousands gathered for a Marriage Equality rally in Sydney that the postal survey was about telling LGBTI Australians they don't have to change, the law does.
"Today is about all Australians coming out and saying for 'goodness sake Australia, let's just make marriage equality a reality'," he said.
Shorten said anyone disappointed with the High Court's decision to green light the postal survey should turn their disappointment "into determination to win".
When asked how many Labor politicians would be voting yes, the opposition leader said he was "reasonably sure" that at least 97% would.