MONTEVIDEO, Uruguay — Uruguayan lawmakers voted Wednesday to legalize gay marriage, making the South American country the third in the Americas to do so. Supporters of the law, who had filled the public seats in the legislative building, erupted in celebration when the results were announced. The bill received the backing of 71 of the 92 members of the Chamber of Deputies present. "We are living a historic moment," said Federico Grana, a leader of the Black Sheep Collective, a gay rights group that drafted the proposal. "In terms of the steps needed, we calculate that the first gay couples should be getting married 90 days after the promulgation of the law, or in the middle of July."