Exactly what Rocci said. I'd say Naruto was somewhere in between. He wasn't joking, but he wasn't completely serious, either, because at that point, he and Sakura certainly weren't official. This is how I imagine it going down in his brain.
Minato: Is this your girlfriend?
Naruto: Uhh... (inside his mind, thoughts going by very quickly: "Well, I'm in love with her and I'm not really sure where she stands in her feelings for me. She's the girl I'm closest to in the world. We have a special relationship and are more than comrades...") Yeah! ("But then again, we aren't really together, so...") That's more or less it!
It was kind of a quick response without too much concrete thought, just spur of the moment response/reaction.
He wasn't too sure about it.