Lisburn City Council has overturned its policy regarding gay and lesbian unions.
Germany introduced similar equality legislation in 2001
The council had banned the use of its wedding room for same-sex civil partnership registrations, prompting gay activists to threaten legal action.
However, after consulting lawyers on Monday night, a council committee decided the ban should be lifted.
Alliance Party councillor Seamus Close said the rules should not have been changed.
"If you attack the nucleus of society namely the family and family values, what's left?" he said.
"I am concerned that the Civil Partnership Act will be used as the lead instrument to bring about changes to other pieces of legislation like, for example, adoption law.
"Where do we go from there, if we allow young people to believe or to think that having two mummies or two daddies is the norm in our society?"
The lawyer's recommendation was considered by a full meeting of the council on Tuesday evening.
A law allowing same-sex couples to enter into a civil partnership comes into effect next month.
However, the council backed a motion in July which stated that same-sex civil partnership registration should not be "afforded the same recognition" as a civil marriage ceremony.
It also stated that the council's wedding room, the Cherry Room, should not be used for such registrations.
The Gay Rights Association said it was "blatant discrimination" and said it would go to court to have it reversed.
Patricia Lewsley from the SDLP said the motion amounted to discrimination, an accusation echoed by Sinn Fein's Paul Butler.
But Ulster Unionist councillors said they were legally entitled to withhold the use of the room.
The Civil Partnership Act creates a new legal relationship, which two people of the same-sex can form by signing a document.
It provides same sex couples with parity of treatment in a wide range of legal matters with those opposite-sex couples who enter into a civil marriage.