Voting for same-sex marriage is a vote for treating all Australians with a "fair go" and aligns with conservative party values, Victoria's Liberals and Nationals yes campaigners say.

Coalition members rallied at the Melbourne Town Hall on Sunday to launch the Victorian Liberals and Nationals for YES campaign for the upcoming same-sex postal survey.

"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," member for Goldstein Tim Wilson told supporters.

"This postal survey is an opportunity to show the traditional Liberal values about commitment and responsibility to the country.

"We urge every single Liberal and National voter across this nation to vote for a change in the law so that a Liberal and National government can change the law and pass legislation by Christmas and create a better Australia."

Former state Liberal leader and now Melbourne Lord Mayor Robert Doyle told the rally people should be free to marry the person they love.

State Opposition Leader Matthew Guy has long supported the yes vote.

"I just hope that is the vote that prevails, but I do hope also for a sensible and mature debate from all sides and the kind of debate that does not oppress each other from having a mature and sensible debate in this country," he told reporters on Sunday.

All Victorians should make sure they cast their vote, Premier Daniel Andrews said as he implored for a yes vote.

He also repeated his criticisms for a postal survey even being held because of the cost and potentially harmful debate, saying parliamentarians are paid "pretty well" to just get on and pass legislation without the ballot.