Finance Minister Penny Wong has told gay rights groups and Labor party members that marriage equality is a "bedrock principle" and its opponents have nothing to fear.
Senator Wong, who helped drive a hard-fought change to the ALP's platform on gay marriage, likened the campaign for marriage reform to the fight for racial equality.
"Equality is a bedrock principle," she said in her keynote address to the annual Rainbow Labor Network conference on Saturday.
"A principle that we could never today contemplate breaching in respect of race, or age, or class or other attribute - but a principle denied to some in the Australian community solely on the basis of their sexuality."
Concerns that same-sex marriage would change the nature of heterosexual marriage were groundless, she told the conference.
"I say to those who oppose marriage equality: there is nothing to fear.
"Marriages between men and women are in no way devalued, nor made less secure by this change."
Australian Marriage Equality campaigner Alex Greenwich told AAP the address was "inspiring".
"Senator Wong gave a very inspiring speech, and I think if she were to deliver a speech like that to the Senate, that she would win over people on all sides of politics," he said.
The South Australian senator described letters penned by church leaders, to be read out during Sunday services, as a disappointing development.
"We're a democracy and obviously people have a right to advocate their view," she told reporters at the Sydney event.
"Obviously not all Australians of faith share the view that we should continue discrimination when it comes to marriage.
"No one is advocating that religious organisations, churches, should be required to solemnise relationships that don't accord with their teaching."
Senator Wong and her partner Sophie Allouache became parents for the first time in late 2011.
She brushed away a suggestion she might get married if same-sex marriage were legalised, calling it "a very personal question".
