The American Psychological Association (APA) will highlight its policy in support of same-sex "marriage" at its annual meeting in August.
The APA also claims it has science that supports that position.
Julie Hamilton, president of the National Association for Research & Therapy of Homosexuality, said the APA cannot have enough evidence on the issue, because there have not been enough reliable studies to make a scientific argument for or against gay marriage.
"The APA is taking sides in a political debate without an adequate scientific basis for their stance," she said. "This is another example of a scientific organization acting politically rather than scientifically."
Caleb Price, research analyst with Focus on the Family, said the vast majority of Americans have "no clue that the mental health profession is heavily involved in activism."
"When groups like the APA position themselves as authorities on a controversial issue in the public arena like same-sex marriage," he said, "they can be disproportionately influential in the debate."