In the U.S., same-sex unions fierce opposition from people who insist that marriage has to be between a man and a woman. In Brazil, an official in the state of Sao Paulo has caused an uproar by giving her stamp of approval to a civil union between a man and a woman ... and another woman. Public Notary Claudia do Nascimento Domingues says the trio live like a family and should be treated as one, while religious groups and at least one legal expert say recognizing the three-way union is immoral and absurd. Will this unique union be allowed to stand? Here, a brief guide:

Who are these people?
They won't talk to the press. (Their union was actually formalized three months ago, but the news just got out.) Domingues says the trio have been sharing bills and living together for three years in Rio de Janeiro, in a relationship filled with "loyalty and companionship." Domingues says she's not "inventing" a new kind of family, merely "recognizing what has always existed." A lawyer who helped draft the civil union document says the women and their man merely wanted to make it official, to protect all of their rights if they split up, and to make it easier to divide pensions, health benefits, and personal property if one or another (or another) dies.

Is this legal?
Domingues insists there is no law prohibiting it. Polygamy is illegal in Brazil, but because these three people won't enjoy all the rights of a married couple, it may not technically count as polygamy. Their civil union document, for example, doesn't bestow any new parental rights to the third person if two members of the group conceive a child. And registering a civil union merely requires establishing that the applicants share an address and a bank account, which these three have done. Brazil has also permitted same-sex unions since 2004, but the law says nothing about allowing a third person to be added to the mix.

Does this arrangement have critics?
Of course. Lawyer Regina Beatriz Tavares da Silva says recognizing a three-way civil union is "absurd and illegal." She's president of the Commission for the Rights of the Family at Brazil's Institute for Lawyers, and says the arrangement is a clear violation of the law against bigamy, even though it isn't a full-fledged marriage. A court, she says, is bound to throw the law out, because a three-way union is "something completely unacceptable which goes against Brazilian values and morals."

How have others reacted?
Religious leaders are outraged. The Catholic Church, which is quite powerful in Brazil and the rest of Latin America, is pretty clear about its condemnation of polygamy as a totally immoral practice. But not everyone is so upset. Look, "a three-person relationship isn't my cup of tea," says Nicole Fabian-Weber at The Stir. But I'm also not a fan of letting governments "tell everyone how they should live their love lives." As long as what these people are doing has "zero" affect on me, "what's the point of getting up in arms?"

Sources: BBC News, Huffington Post, International Encyclopedia of Sexuality, The Stir