Just because there is no "N/A" option on the forms you've encountered where you hang out (USA, right?) that doesn't mean it's universally so. Think globally, act locally!
One new data point for you and our esteemed readers:
In Germany, where I live, "official" forms, when they ask for your religion, give you a handful of "major brand" religions to choose from. After those, the final choice is: verschieden denkend.
Literally, this means "differently thinking," which hits close to freethought, which is an ideology fairly well aligned with Atheism.
However, the meaning in legal and practical terms is this: "I'm not an adherent of any of the above mentioned religions, and thus I claim exemption from Church Tax." Unless you're deep in Bavaria, German officialdom doesn't give a rat's ass about your religious alignment except insofar as it affects your taxation status. In a hospital setting, your religion becomes significant for dietary reasons. They're considerate enough not to serve pork to Muslims. Presumably, if things go very wrong with your treatment and you have no next of kin showing up, this option may also determine the denomination of the cleric brought in to give you the Last Rites or whatever else clerics do with dying or dead patients.