"I just don’t see how non-white characters would fit into my book. All the characters in my head are white."
I see this excuse as a crisis of imagination. Particularly if you’re writing SF, often set in a future when anything can change. When everything can be different than it is now. We’ve already seen our first black president. We’ve seen women in ever more powerful roles. Gays and lesbians are coming out in nearly every corner of society, and universal marriage equality is becoming more and more imaginable.
You can’t imagine a black genetic engineer as your main character? An Hispanic lesbian piloting a starship? Then your imagination needs some revamping. You need to start thinking outside the box. Open up your corner of the world to more possibilities.