"It's about the message it sends to a young me." Ian Thorpe pledges support for same-sex marriage.

Olympic swimming champion Ian Thorpe has joined the campaign pushing for same-sex marriage in Australia, saying it will remove layers of discrimination.

A voluntary postal vote will be held next month after the government’s preferred compulsory plebiscite was rejected for a second time by the Senate.

Thorpe, 34, on Sunday said he believed Australia had thoughtfully come to a position on the matter in recent decades and it was time to make the change.

Listen: We could have avoided this campaign is our PM just did this… (Post continues after audio.)

“The reason why it’s personally important for me is the message it sends to a young me – that the way that I felt was equal to the way anyone else feels,” he told AAP in Sydney.

“I think it’s when we have this kind of recognition in marriage equality that young people can feel that and we start to get rid of all of those layers of discrimination the LGBTIQ community can face.”

The five-time Olympic gold medallist, who became a sporting sensation at just 15, came out as gay after years of speculation about his sexuality.

Thorpe said it was important to promote acceptance to young LGBTIQ people and their families in the face of potentially-damaging messages during the campaign.

“Know that there are a lot of people out there who are supporting you,” he said.

Thorpe believed there would be many athletes supporting the "yes" campaign.

"Sport has taken some great leadership on this," he said.

"Look at the pride rounds we have in the football leagues."

Mamamia has joined the Say No To No campaign. We are proudly on Team Yes and will not be accepting any advertising dollars from those who want to campaign against marriage equality.