SAN FRANCISCO — Gay rights advocates are making plans to get other states to join California in banning psychotherapy aimed at making gay teenagers straight, even as opponents prepared Monday to sue to overturn the first law in the nation to take aim at the practice.

After months of intense lobbying, California Gov. Jerry Brown signed a bill late Saturday that prohibits licensed mental health professionals from using so-called reparative or conversion therapies with clients under age 18. Brown called the therapies “quackery” that “have no basis in science or medicine.”

Two New Jersey lawmakers already are drafting similar legislation, while groups that helped get the California law passed are sharing research, witnesses and talking points with counterparts in other gay-friendly states, said Geoff Kors, senior legislative and policy strategist for the San Francisco-based National Center for Lesbian Rights.

“There are lots of folks today who are looking at this, now that the governor has signed it,” Kors said. “We’ll be reaching out to all the state (gay rights) groups, especially in states that have had success passing LGBT rights legislation.”

The law only applies to licensed therapists, not ministers or lay people who counsel teens to resist same-sex attractions.

Two Christian legal groups, meanwhile, said they would sue in federal court in Sacramento to prevent the law from taking effect on Jan. 1.

The lawsuits will be filed on behalf of therapists whose practices include efforts to help clients change their sexual orientations or reduce their attractions to people of the same-sex; parents who have sought such therapy for their children; and teenagers who currently are undergoing it, lawyers for the California-based Pacific Justice Institute and Florida-based Liberty Counsel said.

Liberty Counsel Chairman Mat Staver said his organization plans to argue in court that the law infringes on the First Amendment and equal protection rights of individuals to give and receive information that matches their personal and professional beliefs.

“What this law does is tell minors that they can no longer receive information about same-sex attractions that they have been receiving and that they find beneficial to them,” Staver said. “It also puts counsellors in a situation where they must present only one viewpoint of this subject.”

The law Brown signed states that mental health providers who use sexual orientation change efforts on clients under 18 would be engaging in unprofessional conduct and subject to discipline by their respective state licensing boards.

Mainstream associations representing psychiatrists, psychologists and social workers have dismissed reparative therapy in recent decades as being ineffective and potentially dangerous to the mental health of teenagers and young adults who are led to believe their interest in same-sex partners is wrong.

As originally written, the bill introduced by state Sen. Ted Lieu, also would have required therapists to warn adult patients of the practice’s risks and limitations and to obtain their written consent before engaging in it.

Lieu dropped the informed consent provision, however, after a number of mental health associations in California — including the California Psychological Association and the California Association of Marriage and Family Therapists — complained that it interfered with the therapist-client relationship.

Both groups, as well as the other leading professional groups, ultimately endorsed the ban for juveniles.

It remained unclear how many practitioners and patients the law would affect.

David Pruden, vice-president of the California-based National Association for Research and Therapy on Homosexuality, a professional association that supports treatment for homosexuality, estimated there are two dozen therapists statewide who engage in efforts to change sexual orientation, and not all of them treat adolescents.

The association plans to be a plaintiff in the Liberty Counsel lawsuit, with its support based mostly on the law’s symbolic effect than its consequences for large numbers of California teens and their counsellors, Pruden said.

“If you said, realistically, how many hamburgers did you think you weren’t going to sell at McDonald’s because of the new pickle law, the answer is not very many,” he said. “Then the question becomes should we be legislating pickles.”

Staver thinks the law could impact hundreds of licensed Christian psychotherapists and their teen clients from religious families nationwide. Depending on how it’s enforced, California therapists who treat clients in other states via Skype, and therapists in other states who conduct telephone sessions with California residents could be investigated for misconduct, he said.