Since 1996, the federal government has defined marriage as between a man and a woman. That could change if proponents of gay marriage succeed before the Supreme Court. For the second straight day, the nation's highest court will hear arguments in a high-profile case on gay marriage, one of the hottest social issues in America. On Tuesday, the justices heard a challenge to California's Proposition 8 ban on gay marriage. Today, they will hear arguments in a challenge to the Defense of Marriage Act, the 1996 law signed by President Bill Clinton that defined marriage as heterosexual and denied federal benefits to gay couples, who are legally married in their state.