I have a fun (if you don't mind awkward) social experiment. The next time you sneeze in a public place, if someone says "bless you", casually reply with "no thanks".
I'm not sure about where you live, but in Tulsa, OK, you get a sideways glance just for saying "salud" or "gesundheight", but after this exchange I've found some interesting responses.
I once had a sneezing episode ( fit) whilst waiting to cross a busy road, a voice behind me said out loud "God Bless you my son.
I turned around to see a Catholic Priest standing behind he in full regalia so my response ( which just popped into my head for some unknown reason) was simply, "Aha, I thought so, I have an allergy to religion, it gives me terrible hay-fever."
I do that with my students, if they say "bless you" I say, "No thanks, I'm good." I did almost tell on myself one time though. A student asked me if "sometimes god answers prayers." I responded with, "It's simply a mathematical coincidence".
The kid then asked if I was an Atheist. Considering that I was in a position to quickly keep moving, I simply didn't respond.
In response, I like to go up to them, cast sideways glances in both directions, looked completely shocked and whisper "You can SEE me?"
Yeah, I've found this to be a losing proposal. Had tried years ago out of curiosity, also because I found it offensive, people presuming I'm Christian. The bulk of the reactions are pissed / confused, not worth the hassle. I've come to accept it as a cultural / societal norm. However, I still always say 'gesundheight' instead, as this technically means "to your health." That makes me feel a bit better about the whole thing.