This is why I think it is about race and why a lot of people who aren't multiracial don't understand that:

16 questions all multiracial people are asked at least once - Us Vs Th3m

"What are you?" White people ask. - The Race Card Project

Six Questions to Avoid Asking People of Color

http://www.drmariaroot.com/doc/50Experiences.pdf

16 Things You Should Never Say To Mixed-Race People [VIDEO]

There's even a book called What Are You? (an astonishingly rude question IMO) which I tried to get my son to read when he was a teen. He wasn't interested.