Prime Minister Malcolm Turnbull has made the conservative case for same-sex marriage, saying it will result in stronger families and more "loving commitment".
Delivering his first speech for the "yes" campaign since the High Court upheld his controversial postal survey last week, Mr Turnbull said he was confident the Australian people would approve the reform and it would then "sail through Parliament" by the end of the year.
Launching the NSW Liberals and Nationals For Yes group in Sydney on Sunday, Mr Turnbull said he was voting "yes" because it was "fundamentally a question of fairness".
The threat to marriage was not gay couples, he said, but a lack of loving commitment that turned many marriages into a "loveless desert".
"Many people will vote 'yes' - as I will - because they believe the right to marry is a conservative ideal as much as any other conservative principle," he said.
"I am very firmly of the view that families are the foundation of our society and we would be a stronger society if more people were married and fewer were divorced. If consulted by friends about marital dramas I always encourage the singles to marry, the married to stick together, the neglectful and wayward to renew their loving commitment, and the wronged to forgive."
The Coalition campaign launch was also attended by NSW Premier Gladys Berejiklian and former state Liberal leaders Nick Greiner, John Brogden, Kerry Chikarovski, Barry O'Farrell and Peter Debnam.
Mr Turnbull pointed out same-sex marriage had been delivered by parliaments, courts and people's votes in many countries, including those most culturally similar to Australia: the US, Britain, Canada and New Zealand.
"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.
"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.
"If the threat to marriage today is lack of commitment then surely other couples making and maintaining a commitment sets a good rather than a bad example."
Mr Turnbull also urged people to argue their case respectfully.