The ordinance, which may be the first of its kind in the country, passed after Davis introduced it at a council meeting last Thursday, reasoning with his fellow councilors that "I don't feel it's the place of government to define a family," he said by phone.
Mayor Joseph Curtatone signed the ordinance into law on Monday, Davis said.
The city had never had a domestic partnership ordinance before, Davis said, unlike Boston, Cambridge, and many other Massachusetts cities that introduced such policies before same-sex marriage became legal in the state in 2004.
The issue arose recently because of the coronavirus pandemic, as Somerville residents in committed relationships who aren't married approached Davis and other councilors with concerns about being able to visit sick partners in the hospital, he said.
The inclusion of relationships between more than two consenting adults was added just before the meeting at the suggestion of Councilor J.T. Scott, according to Davis.
"The ordinance was ready to go, but it just didn't feel right to me. I wanted more input," Davis said.
Scott called and said he had noticed that the language in the ordinance defined a domestic partnership as a relationship between two people.