The threesome, who refuse to speak to the press, say they have been living together in Rio de Janiero for three years and are sharing the bills and other expenses.
"We are only recognizing what has always existed. We are not inventing anything," Ms. Dominques said.
"For better or worse, it doesn't matter, but what we considered a family before isn't necessarily what we would consider a family today."
The move sparked outrage from religious groups and legal authorities such as Regina Beatriz Tavares da Silva, a lawyer who is president of the Commission for the Rights of the Family. Ms. da Silva told the BBC the union was "absurd and totally illegal", and "something completely unacceptable which goes against Brazilian values and morals."
She claims the union will not be permitted to remain in place.
John White writing for the Catholic Vote blog says the union shouldn't surprise anyone.
"Once you start redefining things, there is no going back. Yeah, there may be controversy about the Trio from Rio now, but are we really going to be surprised when in, say 2025 or so, we see a picture on the front of Time magazine of some dude in Vermont standing in front of a preacher, and next to him a mule decked out in the most fabulous de la Renta?" he asks.
"The answer is no, we will not be surprised, because that’s the nature of secular progressivism. Ever expand the limits of morally acceptable behavior, and alter society’s perception of what is good and what is bad. By definition, what was immoral becomes moral, what was bad becomes good, and the trend marches on."
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