The key to marriage is commitment, and if a same-sex couple has that for each other it won't have any impact on the heterosexual marriage of the prime minister.
It's Malcolm Turnbull explanation of why he believes couples of the same gender should be able to wed in Australia.
"The threat to marriage is obviously a lack of commitment, cruelty, desertion," he told the National Press Club in Canberra on Thursday.
"If same-sex couples were able to describe or formalise their relationship as a marriage, we have no doubt that would not undermine... our marriage."
Mr Turnbull and his wife of 36 years, Lucy, will be voting yes in a national vote on same-sex marriage if his government is reinstalled on Saturday.
"We welcome couples making a strong commitment and we are very pleased to support that being described from a legal point of view, as a marriage," he said.
The prime minister has promised to hold a plebiscite on the issue, convinced if the nation votes in favour a same-sex marriage bill will sail through the parliament.
But Labor says the nonbinding vote is a waste of money and could lead to hate campaigns aimed at same-sex couples and their children.
The opposition and the Greens have refused to say whether they will back enabling legislation for a plebiscite.
Mr Turnbull earlier said it would be untenable for Labor to try to block the bill.
The argument that Australians could not be trusted to have a civil discussion on the issue paid "very scant respect for the maturity of our democracy", he said.
But Mr Shorten believes community attitudes had moved and Australians were ready for the parliament to inscribe same-sex marriage into law.
He accused the prime minister of being bound by a deal with the right of his party.
"If he knows the majority of Australians want marriage equality, why are we spending money for any other reason than he has to," Mr Shorten said.
