About 5,000 people have attended a rally in Perth urging Prime Minister Tony Abbott to allow Coalition MPs a free vote on any same-sex marriage bill.

Queensland Liberal National Party (LNP) MPs Warren Entsch and Teresa Gambaro will co-sponsor a private member's bill, to be seconded by Labor backbencher Terri Butler, which is expected to be introduced to Parliament in August.

Among the speakers were advocacy groups, a reverend and politicians, including Greens senator Rachel Siewert and Labor MP Alannah MacTiernan.

Ms Siewert said it was time Mr Abbott started reflecting public sentiment on the issue.

"You only have to look at what happened recently in Ireland and of course in the US to see that he is out of step with not only as I said his community, the Australian community, but also the global community," she said.

"The Bernardis, the Abbotts, the Abetzes - they're wrong.

"Their love is no more important than yours."

Organiser Sally Rugg from GetUp! said it was the biggest marriage equality rally so far this year.

"It's been remarkable," she said.

"This is the fourth in a series of events. We had Sydney, then Melbourne, then Canberra and Perth's outdone them all combined.

"It's absolutely thrilling."