It is unconstitutional to abridge marriage on the basis of sex. In May 2011, national public support for same-sex marriage rose above 50% for the first time. In May 2012, the NAACP, the leading African-American civil rights organization, declared its support for same-sex marriage and stated that it is a civil right. In June 2013, the Supreme Court of the United States struck down DOMA for violating the Fifth Amendment to the United States Constitution in the landmark civil rights case of United States v. Windsor, leading to federal recognition of same-sex marriage, with federal benefits for married couples connected to either the state of residence or the state in which the marriage was solemnized. In May 2015, national public support for same-sex marriage rose to 60% for the first time. In June 2015, the Supreme Court ruled in the landmark civil rights case of Obergefell v. Hodges that the fundamental right of same-sex couples to marry on the same terms and conditions as opposite-sex couples, with all the accompanying rights and responsibilities, is guaranteed by both the Due Process Clause and the Equal Protection Clause of the Fourteenth Amendment to the United States Constitution.

He also stated that his position on same-sex marriage was "evolving" and that he recognized that civil unions from the perspective of same-sex couples was "not enough". On May 9, 2012, President Obama became the first sitting president to support same-sex marriage. He still said the legal question belonged to the states. In October 2014, Obama told an interviewer that his view had changed.

On May 6, 2012, Biden was asked on “Meet the Press” whether his views had evolved on same-sex marriage. The vice president replied the matter “is all about a simple proposition. Who do you love. Who do you love?” “I am absolutely comfortable with the fact that men marrying men, women marrying women and heterosexual men and women marrying one another are entitled to the same exact rights, all the civil rights, all the civil liberties,” Biden continued. “And quite frankly, I don’t see much of a distinction — beyond that.” Biden had just come out for marriage equality at a time when same-sex couples could marry in just six states and D.C. and then-President Barack Obama was still in the middle a years-long evolution on same-sex marriage. During the interview, Biden added he had just visited the home of a same-sex couple in Los Angeles for a fundraiser, where he had an epiphany after seeing the young children the couple was raising. “And I said, ‘I wish every American could see the look of love those kids had in their eyes for you guys,'” Biden said. “And they wouldn’t have any doubt about what this is about.'”

Biden was “very clear and very direct when asked if he was comfortable with gay marriage.” - WaPo

The fallout was immediate. Days later, Obama gave his own interview with Robin Roberts of ABC’s “Good Morning America” (who was closeted at the time) to declare his evolution on same-sex marriage was complete and say he “just concluded that for me personally it is important for me to affirm that I think same-sex couples should be able to get married.”

Biden had taken the lead from Obama and come out first in support of marriage equality.

Following the announcements from Biden and Obama, numerous other public figures — ranging from House Democratic Whip Steny Hoyer (D-Md.) to singer Jay-Z — declared their support for same-sex marriage. The endorsements from the two leaders had the effort of normalizing a position that heretofore was widely considered controversial. Months later, the societal effects were evident with victories in every state where same-sex marriage was on the ballot. Maryland, Maine and Washington State legalized same-sex marriage and Minnesota rejected an amendment that would have made a ban on same-sex marriage part of the state constitution. Nothing like that had ever occurred before. It led to a series of state legislatures legalizing same-sex marriage and preceded the U.S. Supreme Court ruling in 2015 ensuring marriage rights for gay couples nationwide.

Former presidents Bill Clinton, Jimmy Carter, and Barack Obama, former vice presidents Dick Cheney,Al Gore, Walter Mondale, and Joe Biden have voiced their support for same-sex marriage, as have former first ladies Laura Bush, Hillary Clinton, Michelle Obama, and Nancy Reagan.