I don't get what everyone's problem is with "labels." That's what words are for, they label things.
We don't have labels for people who don't believe in unicorns.
This is a false equivalency, because to my knowledge there is no label for someone who believes in unicorns. There are general terms like delusional or insane for instance, but if you want to use that as a label for unicorn believers then one might argue that a label for people who don't believe in unicorns is simply "sane." We live in a world where there are names for people who believe in one or more deities, so why wouldn't there be a word for those who don't? If we frequently encountered people who believed in unicorns we'd likely have a word for it and a word for those who don't believe.
It doesn't seem ridiculous or oppressive or whatever that I be expected to indicate that I'm an atheist when discussing related topics with someone - it's a quick, simple way to give a general idea of my stance. If someone then makes assumptions that are not supported by that broad description, what do I care? If they don't want to be wrong, they'll ask for clarification. If they don't mind being wrong then they'll probably live by their assumptions and I'm no worse off for it. Everyone thinks they're a rational thinker or a skeptic or whatever. So saying that doesn't necessarily tell someone much about you. Saying you're an atheist indicates much more in this case.
In short - I just don't get the fuss about labels, maybe someone can explain it to me. I get why someone might avoid labels that are of insufficient resolution to meaningfully capture some aspect of yourself (i.e. if someone who is a transgender female is attracted to other females, I can understand a hesitancy to identify as either gay or straight) but the label "atheist" is pretty straight forward especially in the context that it takes for most of us here. And, indeed, there are many other words to describe succinctly what one's theistic leanings are that one doesn't have to rely on either being labeled atheist or theist.