Two women and a man have entered into a civil partnership in the Brazilian state of Sao Paulo.
Legal recognition of the three-way relationship follows a decision by the Latin American country to extend civil partnerships to same-sex couples.
Previously, the law regulating “stable unions” existed solely for heterosexual couples who were cohabiting.
It meant that they had rights of inheritance if one of them died and that there were rules on the division of property and estate if a relationship broke down.
But the relaxation of the rules has led to the three people being registered as a stable union in Brazil for the first time after the notary said she could find no objections.
She told local media that she “went to investigate if there was some legal impediment, and I verified that there wasn’t. I couldn’t recuse myself from notarising the declaration.”
The Brazilian Family Institute, which openly supports both polygamous and same-sex unions, welcomed the decision.
“We have to respect the private nature of relationships and learn to live in this pluralistic society recognising different desires,” said the group’s vice-president.
British sociologist Patricia Morgan, who specialises in family policy and criminology, said she was not surprised by the ruling, adding that similar attempts have been made in the Netherlands.
She that the proliferation of a range of relationships that will be legally considered equal to marriage was inevitable once the institution had been redefined.
“In the Netherlands to be equal they opened up civil partnerships to heterosexuals as well as to gays but then found that there were these three-in-a-bed relationships that were seeking legal recognition,” she said.
“I think it is all part of the cause,” she said. “Once you break away from one man and one woman, what do you expect?
“This is the traditional regenerative relationship. Once you allow two men [to wed], where are your boundaries?
“People say this won’t happen but where does it stop? You are going to get polygamy from Muslims, aren’t you?
“People are simply shutting their eyes if they think that this is not going to happen,” she said.
Part of the problem, she added, was the modern view of marriage as a “couple relationship” based on subjective definitions of “love”.
This was to the exclusion of its wider purpose as a public contract serving the common good by supporting the procreation and education of future generations.
In Britain, the Scottish Parliament last month announced that it intended to legalise gay marriage, with the first ceremonies expected to be held in 2015.
The Prime Minister, David Cameron, has said he is “absolutely determined” to bring forward gay marriage laws for England and Wales “in this Parliament”.
The issue of gay marriage has prompted many Conservative Party activists to leave the party or refusing to renew their membership, according to press reports, with membership down by 60 per cent since 2005.
The mainstream Christian churches as well as Muslim, Jewish and Sikh leaders are opposed to the proposals.