There are thousands of irrational things I don't believe: I don't believe the world was created by magic unicorns. I don't believe that murder should be the first recourse for resolving a dispute. I don't believe that only males between the ages of 7 and 14 should receive education. I could go on ... and on ... but, to save boring you, I shall list only one more: I don't believe in any form of deity. So why do so many people think that it is either rational or acceptable to define me by the last of these things?
One of the more insidious trappings of apartheid in South Africa or the One Drop Rule in the United States was the arbitrary classification of humanity into "whites" and "non-whites". The definition of one race in terms of another is itself racist: these terms assume that "white" is the norm against which other racial attributes are measured. It demeans dark-skinned people by euphemising a characteristic they have and defining that characteristic in terms of what it is not. We don't call homosexual people "non-heterosexual" (or heterosexual people "non-homosexual").
An "atheist" is, by definition, a "non-theist", a person who does not believe in deities. Like to the race and sexuality examples above, it defines us in terms of what we are not and ignores what we are. It makes the assumption that some form of theism or religion is the norm against which we should be measured. It is similarly insidious.
I'm not referred to as a "non-murderer" or a "non-discriminatory-educator" (even though they are true), so why should I be referred to as a "non-theist". I'm a humanist, I'm a secular rationalist -- refer to me by those positive things, by things I choose for myself, not by my lack of credulity for superstitious mythology!
Yes, this is important. The coming census will not ask me to define my position in terms of a belief in magical unicorns or a position on the education of males. Quite correctly, there will not be a loaded question: "What kind of murder do you commit?" with response options of subspecies (genocide, uxoricide, drive-by shooting, etc.) and "non-murderer", but there will be one that asks: "What is your religion?" with response options of subspecies and "no religion". The unspoken assumption from this question, entirely unencumbered by such inconvenient things as contrary evidence, is that religion is the norm.
Every time we permit others to define us in terms of religion, by getting us to respond to loaded questions or calling us "atheists", we are playing into their hands by allowing them, instead of accepting us for what we are, to pass off their irrational superstition as the norm. It isn't.