Tony, 48, Kaye, 48, Kevin, 40, and Sandi, 40, who live in San Jose, signed legal documents to give them similar status to actual spouses.
The foursome practice polyamory - the belief people can love several partners at once with everyone's blessing.
The unusual household was formed 15 years ago when Briton Tony and his legal wife Kaye invited their married friends Kevin and Sandi to stay at their home.
The couple decided to stay for good after falling in love with each other.
Three years later, Sandi fell pregnant with Tony's baby and all four changed their surname to his name, 'Luck'.
Kaye said: "When Tony and Sandi confessed they were falling for each other, we all sat down and had a proper conversation.
"All four of us discussed our feelings and decided we should stay together.