U.S. District Judge John E. Jones III's decision Tuesday is a victory for 11 couples, a widow and one of the couples' two teenage daughters who filed the first challenge to the law. Gov. Tom Corbett's office had defended the law after Attorney General Kathleen Kane called it unconstitutional and refused to defend it.
Pennsylvania was the last remaining state in the Northeast to outlaw gay marriage.
Lawyers for the plaintiffs say the law inflicts harm on same-sex couples and their children by depriving them of the legal protections and tax benefits afforded to married couples.
An appeal to the 3rd U.S. Circuit Court of Appeals is likely.
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