A polygamist man in Montana is seeking to marry his second wife after he was inspired by the US Supreme Court ruling that legalised same-sex marriage nationwide.

Nathan Collier, 46, married his first wife, Victoria, in 2000 and now intends to marry his second wife, Christine. They have seven children from previous relationships.

The three went to the Yellowstone County Courthouse in Billings, Montana, last Tuesday to apply for a marriage license.

Bigamy is illegal in the US, including in Montana, and Collier said he will sue if his application is denied.

He married Christine in a religious ceremony in 2007 in order to avoid bigamy charges.

"It's about marriage equality. You can't have this without polygamy," Collier said.

Montana's law states that "a person commits the offense of bigamy if, while married, the person knowingly contracts or purports to contract another marriage unless at the time of the subsequent marriage."

Those convicted of bigamy will be fined an amount not exceeding $500 or will be imprisoned for less than six months.

County clerk officials first denied Collier's application but said this time they would consult with the county attorney.

Kevin Gillen, Yellowstone County chief civil litigator, said, "I think he [Collier] deserves an answer."

Collier said he was inspired by Chief Justice John Roberts' dissenting opinion in the same-sex marriage ruling.

"If not having the opportunity to marry 'serves to disrespect and subordinate' gay and lesbian couples, why wouldn't the same 'imposition of this disability' ... serve to disrespect and subordinate people who find fulfilment in polygamous relationships?" the Chief Justice wrote.

Collier was a former Mormon who was excommunicated because of polygamy. The family appeared on the reality TV show "Sister Wives."

"My second wife Christine, who I'm not legally married to, she's put up with my crap for a lot of years. She deserves legitimacy," he said.

In a Facebook post last July 1, Collier said, "I've still not received a phone call from the county attorney on their decision to grant or deny our license to marry."

"I have been unofficially informed through other sources, however, that they are discussing charging me criminally. I knew the risks I faced when I asked the State to grant legal legitimacy to my family, and I accepted those risks," he said.

He said he is saddened that in the US, which he calls "the land of the free," he had to make such a request.