I'm a little disappointed that you continued to argue with them for so long, there was no point to it other than getting your own kicks then sitting back and chuckling. Why even weigh in on arguments of that nature, it's not necessary when that much stupidity is being thrown back and forth like a volleyball.
Why don't people understand that mixed race people don't always blatantly look mixed race? My best friend is from a mostly black african family, but she's white with blue eyes like you, because her dad is european. The black people in her family can wear whatever they damn well like and nobody minds, but some american guy was actually offended by the idea of her wearing any traditional african clothes at a family party because she doesn't look black enough.