Sorry, but I disagree with this. There is a difference between innocent curiosity and being disrespectful in how one asks questions.
It IS personal when somebody stares at me like I'm a freak or feels the need to ask what my "race/ethnicity/nationality" is. I don't believe that anyone should be obligated to explain their heritage to complete strangers, especially if the question is asked in a disrespectful way.
Just because you don't mind when people ask about your heritage, doesn't mean that others will feel the same way. Personally, I tend to feel uncomfortable when people ask me certain questions because I feel like they're judging me and sizing me up...putting me on the spot because my ambiguous appearance makes them uncomfortable.
I would rather have a conversation with somebody who takes the time to know me instead of immediately jumping into a discussion of what I'm mixed with. My first thought is, why do they want to know? How will that information serve them? I know that people often feel the need to classify others, but I don't want to be put into a box. I don't want to be stereotyped. I want to be acknowledged for who I am, not what people think I am or what they expect me to be.