In an op-ed piece in today's NY Times Tamara Winfrey Harris explains: "Racial identity cannot be fluid as long as the definition of whiteness is fixed."
It's rare that a moral argument has such a precise and simple resolution. You can only go one way if it makes sense to go the other.
Kareem Abdul-Jabbar, who is over 7 feet tall, made a similar analogy in a Time essay. You can allow a white person to self-declare as black, if he can self-declare as 5 foot 8 inches tall.
We may want to live in a world where a black person could decide to be treated as if they were white, but we do not live in that world.
In other words, there are some things about us that are immutable.