"I'm convinced there is a case for change," NSW MP Stephen Jones told ABC Radio on Tuesday, adding it was consistent with core Labor values of equity and fairness.
It was "fundamentally discriminatory" to prevent same-sex couples from marrying.
Mr Jones denied he was defying the party leadership by arguing for change, saying Prime Minister Julia Gillard had given a very clear signal to the Labor caucus she expected all MPs to engage in the debate around ideas.
"That's exactly what I'm doing," he said, adding he would put his views on the issue to the caucus and the party.
However, the MP said he would abide by any decision of the party's national conference or the caucus.
Mr Jones said it was unlikely that change would come about through an "heroic dash to the finish line".
"What we need to do is to bring everyone along with us and that means engaging in a collegiate debate ... that respects the views of all sides," he said.
North Queensland independent MP Bob Katter won't be talking to his electorate about gay marriage, saying he already knows what voters think about the issue.
"Their attitude is not in favour," he told reporters in Canberra.
"If you want to have a relationship you can have it but you can't ask the government of Australia to bless it," he said.
Opposition legal affairs spokesman George Brandis says the coalition would not support any amendment to the Marriage Act to allow for same-sex marriages.
Lower house MPs on Monday night debated an Australian Greens motion that called on all parliamentarians to gauge their constituents' views on the issue of marriage equality.
"It is the power of love that will force this parliament and this country to face the reality of what marriage and love means in the 21st century," Greens MP Adam Bandt said during the debate.
"We want those who are still stuck in the old way of thinking to go out and engage with the people in their electorates to find out where we are now at."
Greens senator Sarah Hanson-Young said she wanted to see all politicians participate in the debate.
"I thank those members who came out in support of the idea of equality," she told reporters.
"It's now time that Julia Gillard and (Opposition Leader) Tony Abbott dropped their opposition to giving their members the opportunity to speak freely and vote freely on (this) issue."
Cabinet minister Craig Emerson said the Labor Party had resolved the issue at its national conference.
"That affirms the position that marriage is a union between a man and a woman," he told Sky News.
People inside the party could take their "legitimate views" about same-sex marriage to the party's national conference, likely to be held in 2011.
Liberal frontbencher Greg Hunt said it was time the Labor Party allowed its MPs to cross the floor in parliament.
"It should drop the blanket ban on MPs having a vote of conscience," he said.
Labor backbencher Deborah O'Neill suggested a conscience vote would be a good idea but until it was allowed she intended abiding by party policy.
"We need to listen to our communities," she told reporters.
Liberal frontbencher Malcolm Turnbull represents an electorate - Wentworth in Sydney's eastern suburbs - that contains a sizeable gay and lesbian population.
He has long been a supporter of removing legal discrimination against the gay community.
"It is always a balance on this," he told reporters, pointing to Liberal Party policy that says marriage is an act between a man and woman.
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