The Anglican priest who implored Bill Shorten not to refer to opponents of same-sex marriage as "haters" has previously compared homosexuality to sex with animals and pedophilia.
The Rev Ian Powell also fears legalising same-sex marriage will open the floodgates to polygamy.
Opponents of a plebiscite are concerned a plebiscite campaign will open the flood gates to hateful and homophobic speech that could be damaging to gay people and their families.
Mr Powell has written on his Facebook page a number of times of his concerns about the "slippery slope" of changing the definition of marriage.
"I am happy to have a large bet that once we do redefine it then very soon we will have all sorts of pushes to redefine the redefinition that will be impossible to avoid," he wrote in March 2015.
"Polyamorists have been quite overt that they want their love recognised."
In June he posted a link to a Guardian story about polyamory, saying it used the same arguments as same-sex marriage advocates to call for acceptance.
"It is weird that this obvious logical step is dishonestly, thoughtlessly or stupidly ignored," he wrote.
In a 2012 lecture on homosexuality and Christianity, he discussed a Bible passage in which God expressly rules out sexual practices when speaking to Moses, including homosexual sex and bestiality.
"I've met people whose deep innate longings sexually is to have sex with animals," he says.
"The first man I ever talked to about this he felt it as deep as any other sexual longing. As much as I would like to have sex with women he wanted to have sex with animals, that's just the way he found himself.
"Why does he not do it? Well, he understands that's not what the Bible says."
Mr Powell says the argument that Jesus never said anything about homosexuality so it's not banned is "ridiculous".
He accosted Mr Shorten as the opposition leader left a parliamentary church service in Canberra on Tuesday morning.
He referred to a comment Mr Shorten made after the Orlando nightclub shooting that the plebiscite campaign would "give haters the chance to come out from under the rock".
"Please don't speak like that about other Australians so we can have a civil and tolerant discussion rather than the hate that's been coming," he told Mr Shorten.
The Labor leader responded by saying people of faith could be opposed to marriage equality, but some who objected to same-sex marriage did have homophobic attitudes.