A majority of the Supreme Court on Wednesday questioned the constitutionality of the 1996 Defense of Marriage Act and whether it created unequal classes of married couples by extending federal benefits only to marriages between a man and a woman.

Justice Anthony M. Kennedy, thought likely to be the deciding vote as the court held its second day of hearings on same-sex marriage, told the advocate defending the law that it did not really promote “uniformity” in federal law.