Obama interview on same-sex marriage to air The complete interview airs Thursday morning on "Good Morning America" "No president could have done this 10 years ago," Rep. Barney Frank says "I believe marriage is a relationship between a man and a woman," Mitt Romney says (CNN) -- Another flurry of reactions is likely to follow Thursday when a full interview airs with President Barack Obama endorsing same-sex marriage -- the first American president to do so. "At a certain point I've just concluded that for me, personally, it is important for me to go ahead and affirm that I think same-sex couples should be able to get married," Obama said in an interview with ABC's Robin Roberts. Excerpts aired Wednesday evening on "World News With Diane Sawyer," but the full interview will appear on ABC's "Good Morning America." Obama once opposed such marriages. "I had hesitated on gay marriage, in part, because I thought civil unions would be sufficient," the president said. I was sensitive to the fact that -- for a lot of people -- that the word marriage is something that provokes very powerful traditions and religious beliefs. But, Obama said, his thinking shifted as he witnessed committed same-sex marriages and thought about U.S. service personnel who were "not able to commit themselves in a marriage." In 2011, the Pentagon stopped enforcement of the "don't ask, don't tell" policy on homosexuals serving in the military. That change played a part in Obama's announced stance on same-sex marriage. "When I think about those soldiers or airmen or Marines or sailors who are out there fighting on my behalf and yet feel constrained, even now that 'don't ask, don't tell' is gone, because they are not able to commit themselves in a marriage, at a certain point I've just concluded that for me personally it is important for me to go ahead and affirm that I think same-sex couples should be able to get married," he told ABC. The announcement puts Obama squarely at odds with presumptive Republican presidential nominee Mitt Romney, who on Wednesday said during an appearance in Oklahoma, "I believe marriage is a relationship between a man and a woman." My view is the domestic partnership benefits, hospital visitation rights and the like are appropriate, but that the others are not. The issue is a divisive one in American politics, but it's uncertain how the development might play out at the voting booth. Obama said he was "disappointed" by Tuesday's vote on the issue in North Carolina, which he described as discriminatory against gays and lesbians, a spokesman said earlier Wednesday. The Family Research Council criticized Obama, and its president said on CNN's "The Situation Room" that the decision will aid Romney. "The president, I think, has handed to Mitt Romney the one missing piece in his campaign," said Tony Perkins. That is the intensity and motivation that Mitt Romney needs among social conservatives to win this election. An expert on religion and politics said the move will make "an already close election even closer." "President Obama's words today will be celebrated by generations to come," Chad Griffin said in a statement. Griffin later told CNN's "Piers Morgan Tonight" that "we will never have another president, Democrat or Republican, that opposes gay marriage." Same-sex marriage foe Bill Donohue, president of the Catholic League, told Morgan that "a child needs a mother and father." Barney Frank, a gay Democratic congressman from Massachusetts, appearing on CNN's "Erin Burnett OutFront," said that "no president could have done this 10 years ago." Before Tuesday's North Carolina referendum, 30 states had voted in favor of constitutional amendments that seek to defend traditional definitions of marriage as a heterosexual union. Six states -- Massachusetts, Connecticut, Iowa, Vermont, New Hampshire, New York -- and the District of Columbia issue marriage licenses to same-sex couples. CNN's Phil Gast, Jessica Yellin, Bill Mears, Dan Gilgoff and Jim Acosta contributed to this report.