The 36-year-old actor said he took the nearly-nude photo and posted it on Instagram just for fun, reported Ace Showbiz.
"It is what the people want. There's a lot to get straight here," the actor said on the David Letterman show.
"I don't even know if we have time. But to me, it's just a fun thing. It's something I don't put a ton of thought into, but it gets a lot of attention. I have a lot of followers on Instagram," he said.
When David Letterman showed him the risky picture that had since been deleted from the social media, Franco said, "Awful... Like that Britney Spears shaved head moment, where it's sort of like, 'All right, you want me Here you can have me, but I am going to be really ugly'. You know what I mean"