The actor’s mother, Jane Pitt, threw her support behind Mitt Romney earlier this year and called on Christian voters to accept the Republican, who is a Mormon.
She said she would be happy to vote for a family man with high morals who shares Christian conviction on homosexuality over the likes of Obama.
But Pitt told Telegraph that the American president retains his vote.
He said: “I am an Obama supporter and I’m backing his election campaign. Gay marriage is inevitable. The next generation, they get it. It is just a matter of time before it becomes a reality.”
Earlier in 2006, the star of Ocean’s Eleven said that he would not marry Angelina Jolie until America allowed same-sex unions.
The couple announced their engagement in April. The following month, Obama declared his support for homosexual marriage.