The managing director of the Australian Christian Lobby (ACL) Lyle Shelton has claimed he believes vile homophobic posters that were reportedly seen in Melbourne's CBD are part of a conspiracy by marriage equality supporters in order to make a point about the validity of the postal plebiscite process.

In an interview with 'The Project' on Monday night, presenter Waleed Aly questioned Shelton on whether it was naive to believe a "civilised debate" could be had on marriage equality, given the attention the posters have received.

In response, the ACL leader said the controversial image "obviously doesn't come from us," and said the posters may have been planted on purpose by same-sex marriage advocates looking to critique the idea of a "respectful debate".

The poster in question is headed by the screaming headline "STOP THE FAGS" and pictures a young child cowering before two rainbow-coloured belts, seemingly about to be punished.

"I don't know who put that poster up. I can't imagine anyone from our side would do that. It may have even come from people wanting to prove the sort of point you're making [Waleed]," he said.

"I know the people on our side of the debate and we're not like that and nor would we be like that.

"You won't see the leaders of our side doing that sort of thing. You will see the leaders of the other side encouraging violence against our side. Death threats against venues where we're holding meetings."