In the interview, Obama said, "I have hesitated on gay marriage in part because I thought that civil unions would be sufficient." He added, "I was sensitive to the fact that for a lot of people the word 'marriage' was something that invokes very powerful traditions, religious beliefs and so forth." Now, he said, "it is important for me personally to go ahead and affirm that same-sex couples should be able to get married."
If you're interested, you can watch a play by Martin Sheen, Brad Pitt, George Clooney etc.
The play is made up of the transcript in a court case that contests a ban on gay marriage in California.