Obama's GOP opponent Friday said Obama is trying to have it both ways on the issue, and Republican Alan Keyes accuses Obama of "deceiving the voters."
Throughout the campaign, Obama has consistently said he favors civil unions.
During a taping of WBBM Radio's "At Issue" program Friday, Obama was asked his personal views. He said he personally believes marriage should be between a man and a woman, but he added that he doesn't understand people who say gay marriage threatens the vows straight couples take.
Keyes supports a constitutional amendment banning gay marriage and has made the issue a focus of his campaign.