What should I do if I asked him if he was asian without thinking?
Why did you ask him if he was Asian?
I heard that the average penis length of an asian is small, so I wanted to test him about that.
You should think before asking such things that involve racial stereotypes. It's rude to ask someone you aren't close to about such private topics.
I was just curious.
It's not okay to let your curiousity cause you to be disrespectful. You should show others the same respect and dignity that you would want them to show you.