A routine House Judiciary Committee report backing the Defense of Marriage Act helped sway Congress in its favor 17 years ago. But on Wednesday, that same report drew gasps when Justice Elena Kagan read key excerpts. “Congress decided to ‘reflect and honor a collective moral judgment’ and to express ‘moral disapproval of homosexuality,’ ” Kagan said, provoking an audible reaction from the audience. It was a dramatic moment in the closely watched deliberations over a key section of the Defense of Marriage Act, which bars the federal government from recognizing same-sex unions. But it was also a moment that underscored how drastically the tenor of the debate over gay marriage and homosexuality has changed since then, not only in public opinion but in official circles in Washington.