Instead of telling us on here, I would've directly turned around and told the people who said that, "That's a cute assumption, but no....we are not a couple. He's my father." And made them feel like the real dummies that they are. That also would've ensured that they think twice next time before commenting.
Other than that, who cares? I wouldn't put too much thought into it.
I go out with my dad sometimes, and I always pray to God that no one thinks we are in a relationship. On the other hand, it does not matter what I think because I cannot control other people's thoughts. People are going to think what they want to, regardless. I live in South Florida, so I see older men with young young girls all the time. I have become immune to seeing it. Of course, in my head that I cannot control, I always wonder whether they are family or a couple. Although, I would never say anything out loud just in case I assume wrong.