Obama is caught up in semantics, apparently believing that gays and lesbians should be allowed to engage in civil unions with all the rights of marriage, as long as they aren't called marriages. That's an evasion that was rightly rejected in May by the California Supreme Court when it overturned a previous ban on same-sex marriage, because such semantic distinctions tend to cast doubt on a union's legitimacy.
At the time of Obama's birth in 1961, some states would not have allowed his interracial parents to marry. He, of all people, should know better.