Gay marriage was the topic of table conversation at Ms Gillard's official Canberra residence on last night when she dined with the brothers and three same-sex couples.
GetUp and Australian Marriage Equality bought the dinner date for $31,000 at a charity auction last year.
There are currently three gay marriage private member's bills before federal parliament, aimed at changing the legal definition of marriage as a union between a man and a woman.
At the ALP national conference in December, Labor adopted same-sex marriage as part of its party platform and also decided government MPs could have a conscience vote on the issue if it came before parliament.
However, Ms Gillard has said she does not support changing the definition of marriage.
Sydney family Sandy Miller, Louise Bucke and their sons Matthew 12, and Dylan 9, will be among those attending the dinner.
Ms Miller said her family wanted to share its experiences of being a two-mum family with Ms Gillard, who has a partner, Tim Mathieson, but is not married.
"We want her to understand our point of view, how it hurts us in our everyday lives," she told reporters in Canberra.
The couple got engaged two years ago.
Their son Matthew told reporters he was a little bit nervous about meeting the prime minister.
"To other people it would make us normal and accepted if our mums could get married," he said.
"I'm proud of my mums. They have gone through a lot."
Matthew said he wanted Ms Gillard to explain why there's a law stopping his mums getting married.
"It should be families are families, no matter how big or small, no matter what shape or size," Matthew said.
John Dini and Steve Russell are butchers and live in Ms Gillard's outer Melbourne electorate of Lalor.
Mr Dini said that from a financial perspective, same-sex marriage would make business arrangements simpler.
"We own a business and property together, and my name is not on any of that," he said.
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