A provocative same-sex marriage mural depicting Tony Abbott and Cardinal George Pell has become the unlikely meeting point for a Catholic prayer vigil in Sydney's progressive inner-west.

The painting, by local artist Scott Marsh, was completed behind Newtown's Botany View Hotel to celebrate this week's result of the same-sex marriage postal survey.

It featured the former prime minister dressed as a bride with his hands down the pants of a shirtless Cardinal Pell, and was titled The Happy Ending.

But it left many Sydneysiders unhappy, and three men were so offended they painted over it in black on Thursday night.

On Friday evening, at least 20 Catholics gathered at the mural to pray the rosary together.

Andrew, a 33-year-old member of the group who declined to give his last name, said the traditional marriage supporters weren't only praying for the cardinal.

"We don't hate anybody," he told AAP.

"We were just here to pray peacefully and to pray for those we disagree with."

Andrew, from Sydney's west, said the group's members were from all different areas.

"I'm all for freedom of speech - I think it's what makes a country like ours great," he said.

"But I also think it has to have a basic level of common decency and respect for one another."

He said he wouldn't want young children, such as his niece, exposed to the material.

A video of the men defacing the mural posted online shows a woman challenging their actions.

The men say the painting is "offensive to the general public" and "pornography".

"Open your eyes, woman," one says.

Long-time Botany View Hotel employee Griffin Hall said they'd been inundated with phone calls and Facebook posts from people who didn't like the artwork.

"It's an absolute shame," Mr Hall told AAP.