Prime Minister Kevin Rudd is an "opportunist" using gay marriage as a selling point with young voters, activists say.
Mr Rudd has changed his position to support gay marriage and raised the possibility of referendum on the matter.
Speaking at a Sydney rally organised by Community Action Against Homophobia (CAAH), gay rights activist Rachel Evans said it was "a smart move" for Mr Rudd to support same-sex marriage because many young people back the issue.
"But he's an opportunist, because actually if he was genuine, he would pass a bill," she told AAP.
Co-convenor of CAAH, Cat Rose, said it was "insulting" to be used to gain votes, while another protester, 17-year-old Mia Sanders said Mr Rudd was treating marriage equality "like a carrot on the end of a stick to get the votes".
The CAAH is calling on Mr Rudd to promise Labor party support for marriage rights.
A rainbow crossing was chalked on pavement outside the NSW ALP offices in Sussex Street Sydney on Tuesday, with at least 25 supporters calling for "homophobia to be nipped in the bud".
The protesters attempted to deliver petitions of 15,000 signatures in favour of legalisation to the ALP front desk.
Access to the offices was denied however and ALP officials refused to be photographed accepting the petitions in the lobby once police had arrived.
Ms Rose said it was "bizarre" to see the Labor party "closing the door" on gay marriage petitions.
"We're pretty concerned that this isn't an issue they've received with open arms," she added.