"I struggle with this," he said. "I have friends, I have people who work for me who are in powerful, strong, long-lasting gay or lesbian unions, and they are extraordinary people, and this is something that means a lot to them and they care deeply about."
Americans divide about evenly on gay marriage, according to the most recent ABC News/Washington Post poll.
Forty-seven percent said it should be legal, 50 percent illegal, with strong opinions on both sides.
As recently as 2006, 36 percent favored legalizing same-sex marriage with 58 percent opposed.
Gay civil unions, with "the legal rights of married couples in areas such as health insurance, inheritance and pension coverage," are less controversial -- 66 percent in favor, according to the latest poll, a new high by a substantial margin.
ABC News' Ariane de Vogue and Gary Langer contributed to this report.