"I've said this many times before - the threat to marriage is not gay couples.
It is a lack of loving commitment," Mr Turnbull told the NSW Liberals and Nationals for Yes campaign launch in Sydney.
"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 did not attend the Yes rally up the road outside Sydney's Town Hall, where the opposition leader was passionately advocating Australians to vote in favour of same-sex marriage.
"What this world needs, and what this country needs, is we need help to maintain families, we need help to raise children, and that is why we need marriage equality," Mr Shorten said.
"We've got one last mountain to climb to make marriage equality a reality; let's climb it together."
The Sydney and Brisbane events were among the biggest rallies seen for some time.
In Sydney, the rainbow crowd waved signs such as "Let's end the hate", "Did I vote on your marriage?" and "How is this not a thing yet?"
In Brisbane, some held signs - such as one that read "Against gay marriage?
Don't marry one" - while others brought along brightly decorated cardboard unicorns and peace signs.
But a chorus of boos rang out from the crowd when a small opposing group arrived bearing signs reading: "The wicked shall be turned into hell and all nations that forget God".
Police said tensions eased when the same-sex marriage rally responded with an impromptu rendition of Gloria Gaynor's 'I Will Survive'.
In NSW, Victoria and South Australia, Liberals and Nationals for Yes were launching their campaigns saying support for marriage was in line with conservative values.
"This postal survey is an opportunity for Australians to show our nation the type of country we want to be - one built on commitment, responsibility, and treating all Australians with a fair go," gay Liberal MP Tim Wilson told
supporters.
The survey forms with the question "Should the law be changed to allow same-sex couples to marry?" will be mailed to voters from this Tuesday.
They'll have the option to tick a "yes" or "no" box and the result will be announced on November 15.
Mr Turnbull said if the majority of Australians voted yes like him, a private member's bill would be presented which would "sail through the parliament".
