Labor backbencher Susan Close has introduced a bill to the South Australian Parliament seeking to allow same-sex marriage.
Dr Close said the traditional rules of marriage between a man and a woman were too narrow and it was time for change.
Her brother is gay and she said it was unfair he was unable to share the same rights as her to marry a partner.
"I've always found it quite affronting that I'm able to get married and he isn't," she said.
Dr Close told the Legislative Assembly the current laws were discriminatory.
"The passage of this law would show that large group of people that we in this place do not believe sexuality to be a basis of exclusion from the simple and fundamental act of choosing to marry," she said.
"The compassionate, human value of that decision cannot be understated in its magnitude, because there are a great many people who by virtue of the discriminatory nature of our current laws are being made to feel lesser."
The Opposition Liberals have a conscience-based policy on the issue, but shadow attorney-general Stephen Wade said, while he supported the spirit of the legislation, it was at odds with federal marriage laws.
"Our MPs are free to express their views for or against same-sex marriage," he said.
"Personally I support non-discrimination in respect to recognition, but I'm a member of the State Parliament, I've got to respect constitutional law.
"As a matter of constitutional law, marriage is the domain of the Federal Parliament so our party won't be supporting laws that we don't think our Parliament has the power to make."
Upper House Labor Government minister Ian Huntersaid any change in the law would be likely to face a High Court challenge.
"There are competing legal views about this, some constitutional lawyers say the state does have the capacity to legislate here, the state never gave up its right to legislate in marriage of course, but we do have an ability to pass legislation and it can be tested in the High Court," he said.
A day earlier, the state Upper House narrowly passed another Labor MLC's motion congratulating New Zealand on its changes to marriage laws.