SAN FRANCISCO (AP) – Two groups opposed to same-sex marriage can join the defense of the state law that defines marriage as a union between a man and a woman, a San Francisco judge has ruled.

The Campaign for California Families and the Proposition 22 Legal Defense Fund said state Attorney General Bill Lockyer can’t be trusted to vigorously defend the law against constitutional challenges currently in the court.

The organizations want to argue that same-sex marriage would harm traditional families.

While he allowed them to join the defense, Superior Court Judge Richard Kramer said he doubted factual disputes were relevant to the legal question before him: whether the marriage law discriminates unconstitutionally on the basis of sex or sexual orientation.

Twelve same-sex couples and the city of San Francisco sued to challenge the marriage law after a California Supreme Court ruling in August that voided nearly 4,000 same-sex marriages performed in San Francisco in February and March.

The state law banning such unions was passed by the Legislature in 1977 and reaffirmed in 2000 by voters’ approval of Proposition 22.

The justices ruled only that San Francisco exceeded its authority and said they would not decide the constitutionality of the state marriage law until lower courts reviewed it.

Kramer’s decision allows the two groups opposed to same-sex marriage to take part in all hearings and appeal adverse rulings.

“The members of the Proposition 22 organization are uniquely qualified to defend this law,” Robert Tyler, an attorney for the Proposition 22 group, said in a statement. “They will seek to prevent any activist organizations from thwarting the will of Californians.”