BATON ROUGE, La., Oct. 5 (AP) - A state judge on Tuesday threw out a Louisiana constitutional amendment banning same-sex marriage, less than three weeks after voters overwhelmingly approved it.

The judge, William A. Morvant of the 19th Judicial District Court, said the amendment was flawed because it had more than one purpose: banning not only gay marriage but also civil unions.

Groups that oppose same-sex marriage expressed outrage, and Michael Johnson, a lawyer for supporters of the amendment, said he would appeal.

"We have judges acting in arrogance to usurp the actions of the Legislature and deny the voters of Louisiana who voted overwhelmingly to support the protection of marriage," said Tony Perkins, president of the conservative Family Research Council in Washington.

A gay-rights group called Forum for Equality had challenged the amendment, arguing among other things that combining the issues of same-sex marriage and civil unions in one ballot question violated state law.