I think the whiteness of sci-fi/fantasy is kind of a mimetic thing/vicious cycle. Theory:
A) Nerdy white d00d(ette)z make SF&F novels.
B) Other nerdy white d00d(ette)z buy and read said novels. Some of them become writers and produce their own.
C) Cycle repeats.
Obviously you have Octavia Butler and others, but this is merely my attempt at esplaining the whiteness of the authordom/fandom.
As for why so many CHARACTERS in SF&F are white, I think it's the "unexamined white male privilege" thing you mentioned. It's the first thing that comes to their mind.
Of course, if I am talking out of my ass (again), please inform me.
(On another note, I now mentally lump Heteronormativity and Default Fantasy Caucasianism into one phenomenon that I call Vanilla Cupcake Syndrome.)