Are you suggesting that being atheist is unnatural?

Of course it is unnatural or do you believe apes have strong opinions about theism? Same goes for theism. Naturality is mostly irrelevant for complex sociocultural views, IMO. I find atheism beneficial, though.

I think this puts a point on your confusion with the descriptor.

Do you believe ducks are convinced a god exists? If not, they’re atheist.

Are rocks convinced a god exists? I’d argue they aren’t sentient and thus not able to - they’re atheist.

Atheism doesn’t require an act of will, isn’t an identity, it only describes one particular thing (which we have a need to describe as religious people get all tizzy about it), just like “blue”, “tall” or “dizzy”. And to belabour the point, it actually describes the absence of a thing, and thus covers all options but one.

An analogous term for someone not believing in aliens could be analienist, you can be analienist regardless if there are aliens or not (as it only addresses the belief). It doesn’t also mean you’re anything else (like tall, handsome, or mysterious). It doesn’t require you to campaign against aliens, throw rocks at the sky, or go to analienist meetings.

As long as you don’t believe in aliens, you’re analienist.

one could frame my question also as what remains in atheism without anti-religion.

A lack of belief in gods. Which is what atheism means. You can be an atheist and pro-religion. My mother is an atheist but still goes to temple every week.

You really need to understand the meaning of the word.

A = lack of

Theism = belief in a god or gods.

That’s all it is. A lack of belief in any gods.