The American Civil Liberties Union is suing Pennsylvania to overturn the state's ban on same-sex marriage.
The move comes two weeks after the U.S. Supreme Court struck down parts of the federal Defense of Marriage Act that limited marriage to only between a man and a woman.
The ACLU wants to legalize gay marriage in Pennsylvania and force the state to recognize the marriages of homosexual couples who wed in other states.
Opponents say gay rights activists are using the courts to push their agenda rather than letting the people decide.
Thomas Peters, with the National Organization of Marriage, said gay rights activists only support the voice of the people when they think it will go their way.
The ACLU said it also plans to challenge the same-sex marriage ban in Virginia.