my own methodology when asked whether I am an atheist or agnostic, is to ask the person inquiring to define what they mean by these terms before I answer.
You need a word to describe your self that doesn't have theism embedded in it.
What's wrong with materialist or naturalist?
It's not about describing yourself: "atheists" can "be" a lot of things beside "atheists".
Philosophically the notions of "nature" and "matter" are nearly as problematic to me as the notion of "god". From this perspective I find it comparatively easier to accept a negative label like "a-theist" (which I mean simply as a honest confession that I do not suscribe to the belief usually called "theism") than a positive one.
Of course, if you actually construe "materialism" and "naturalism" as mere negative notions (meaning dis-belief in a "spriritual" or "supernatural" realm, or dimension) I can accept those labels, too. But in that case they don't express what I positively believe either.