In 2008, when Obama was pretending to be a Christian, he implied that God was against same-sex marriage.
August 18, 2008
When asked to specifically define his views on marriage, Obama stated that he believes “that marriage is the union between a man and a woman.” “Now, for me as a Christian, it’s also a sacred union. God’s in the mix,” he added.
9 May 2012
Barack Obama speaks out and declares support for same-sex marriage
President speaks about issue in deeply personal terms, and affirms his unequivocal support for same-sex couples to marry
In other words, his pollsters told him that he could gain a few votes against Romney by breaking his 2008 campaign promise.