WASHINGTON — A majority of the justices on Wednesday questioned the constitutionality of the Defense of Marriage Act of 1996, as the Supreme Court took up the volatile issue of same-sex marriage for a second day. Justice Anthony M. Kennedy, widely considered the swing vote on the divided court, joined the four liberals in posing skeptical questions to a lawyer defending the law, which defines marriage as the union of a man and a woman for the purposes of more than 1,000 federal laws and programs. “The question is whether or not the federal government under a federalism system has...