Influential union leader Joe De Bruyn has slammed Labor for "pandering" to the Greens on gay marriage and says supporting a law change will not yield more votes for the party.

Mr De Bruyn is the head of the Shop Distributive and Allied Employees Association and sits on Labor's national executive.

Yesterday, a Greens motion urging MPs to speak to the community about gay marriage was passed in the House of Representatives with the support of Labor.

The motion's passing came after increasing tensions within Labor over the issue, with some MPs publicly challenging the party's refusal to support gay marriage.

Speaking on Radio National this morning, Mr De Bruyn accused Labor of abandoning the middle ground, drifting to the left and being in "danger of misunderstanding what they should be doing in federal Parliament".

He says he decided to speak out because he believes Ms Gillard needs some "guidance" and his union had last month resolved to campaign against gay marriage and re-instating euthanasia laws.

Mr De Bruyn says support for the Greens motion has guaranteed continual focus on an issue many Australians are not interested in.

"If I had been Julia Gillard this week I would have put the issue of gay marriage to the vote, killed the issue off once and for all and gotten on with the business of the real issues the ordinary person in the electorate cares about," he said.

Mr De Bruyn says the more Labor "panders" to the Greens the more it will lose the middle ground, making it impossible to win elections.

"There are no votes for Labor in the marginal electorates in the big metropolitan areas on the gay marriage issue," he said.

"We all know that marriage is between a man and a woman not just because that is what the definition of the Marriage Act says but simply because this has been the way that it has been in the existence of the human race."

Health Minister Nicola Roxon says she is not worried about Mr De Bruyn's comments.

She says any policy changes will be thrashed out at next year's national conference.

"We are a major party with established processes," she told Radio National.

"This has been an issue within our party with differing views for a long time."