CNN's Anderson Cooper immediately recognized the significance of the moment.
"For a president who only recently, to use his word, evolved on the issue of same-sex marriage, he made very forceful statements in this inaugural address, actually, historic statements on equal rights for gay and lesbian Americans," said Cooper, who is openly gay.
The sweeping endorsement stands in contrast to Obama's opposition to marriage equality through most of his first term -- until just last May. The change comes amid rapidly growing support for gay rights, including marriage, as reflected in public opinion polls.
Gay rights groups were thrilled by Obama's explicit embrace of their cause.
"By lifting up the lives of LGBT families for the very first time in an inaugural address, President Obama sent a clear message to LGBT young people from the Gulf Coast to the Rocky Mountains that this country's leaders will fight for them until equality is the law of the land," said Chad Griffin, the president of Human Rights Campaign. "As the merits of marriage equality come up for debate from state houses to the halls of the U.S. Supreme Court, and a broad majority of Americans are standing up for liberty and fairness, the President's unequivocal support for equality is a clarion call that all Americans should receive with celebration."
"We were honored that the President included Stonewall among the historic events in American history that have made our union stronger," Griffin said.
Evan Wolfson, president of Freedom to Marry, said his group "applauds our president and the moral leadership he has shown, the moral leadership we will continue to need until all Americans, all loving couples, all families, can share fully in the American promise we celebrate on Inauguration Day."