Allowing same-sex marriage will open a “pandora’s box”, undermining the institution of marriage to lead to the possibility of “polyamory”, the love of multiple partners, Tasmanian Senator Eric Abetz said.

He said his colleagues, who are being lobbied to support a free vote on same-sex marriage, should not be swayed by “the latest fad”.

“If you undo the institution of marriage by redefining it for the latest movement or the latest fad you will open a Pandora’s box for all sorts of other potential possibilities,” he said on Sky News on Thursday.

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Prodded to specify those possibilities, he said “polyamory, clearly – well, polyamory is one of those”.

Mr Abetz pointed to other jurisdictions where polyamory has been pursued, like Holland, “Scandinavia” and the United States.

“Let’s not be under any illusion that once you start unpicking the definition of marriage there will be other consequences,” he said.

He might be right

A Montana man says he was inspired by last week’s US Supreme Court decision legalising gay marriage to apply for a marriage licence so that he can legally wed his second wife.

Nathan Collier with partners Victoria and Christine. Photo: Facebook

Nathan Collier and his wives Victoria and Christine applied at the Yellowstone County Courthouse in Billings on Tuesday in an attempt to legitimise their polygamous marriage.

Montana, like all 50 US states, outlaws bigamy — holding multiple marriage licences — but Collier said he plans to sue if the application is denied.

“It’s about marriage equality,” Collier said on Wednesday.

“You can’t have this without polygamy.”

County clerk officials initially denied Collier’s application, then said they would consult with the county attorney’s office before giving him a final answer, Collier said.

Yellowstone County chief civil litigator Kevin Gillen said he is reviewing Montana’s bigamy laws and expected to send a formal response to Collier by next week.

“I think he deserves an answer,” Gillen said, but added his review is finding that “the law simply doesn’t provide for that yet”.

The Supreme Court’s ruling on Friday made gay marriages legal across the country.

Chief Justice John Roberts said in his dissent that people in polygamous relationships could make the same legal argument that not having the opportunity to marry disrespects and subordinates them.

Collier, 46, said that dissent inspired him.

He owns a refrigeration business in Billings and married Victoria, 40, in 2000.

He and his second wife, Christine, had a religious wedding ceremony in 2007 but did not sign a marriage licence to avoid bigamy charges, he said.

Collier said he is a former Mormon who was excommunicated for polygamy and now belongs to no religious organisation.

A bipartisan bill sponsored by Liberal and Labor backbenchers to legalise same sex marriage in Australia will reportedly go before Parliament in August.

Prime Minister Tony Abbott has talked down the proposal, pointing out that it is uncommon for private members’ bill to succeed.