Among the keynote speakers, the couple said having their marriage taken away when the High Court struck down the ACT law was "mortifying" and left them "gutted".

When many protests leaders in Canberra's Garema Place often struggle to gather a crowd, thousands struggled to fit into the square on Sunday to rally for a free vote on gay marriage.

The young and progressive rubbed shoulders with older suburban mums and dads, not ready to marry but keen for their children to have the choice.

Samantha Wilson said the recognition from the state was important for them and many other LGBT couples.

"We were a bit surprised about how good it felt to get that piece of paper in the mail. To be recognised by our country and it's law.

"We were overwhelmed by the support and we are still being congratulated to this day," she said.

Ms Wilson said she was shocked at how devastated they were by the High Court ruling.

"It was quite an unusual feeling to have that taken away after just a few days. We knew we were going to feel sad, but we felt mortified and gutted that a government could take that away from us.

"And now it's like us feeling like we are the minority, like we are lesser and we know we are not, we want to be equal," she said.

Ms Wilson said they wanted to share their story so the country knew "normal" Australians were being hurt.

"We have been up since 3am with a small baby, we are the same as your neighbours. We are normal people who are in love and are a solid family unit who are as entitled to marriage as anybody else," she said.

Rally-goer Amanda Charles, of Hackett, said she had seen Canberra gay marriage rallies get bigger, and the issue was about choice.

"I'd like my children who are born of a gay partnership to have the choice to get married in the future, and to be proud of their parents, if their parents choose to get married," she said.

Speaking at the event, Deputy Director of Australian Marriage Equality, Ivan Hinton-Teoh, said the crowd had even outdone a similar event held recently in Sydney.