Is San Francisco a culture shock to small town America?

It can be. I've had relatives from Tennessee who visited and felt overwhelmed.

One visited around 1998 and had never seen "valet parking" before until he got here. Another was overwhelmed by the amount of homeless people and aggressive panhandling we have going on (they often target tourists.)

Our public transportation has instructions audibly delivered in at least three different languages all day long - this would be unusual for them.

Traffic is routinely stopped for some kind of peace rally or protest or bicycling takeover of the streets but it never really upsets you if you've lived here a while because it's just a regular occurrence here. You get some unusual street performers here and raving people with megaphones totally yelling at you about Jesus or their cause du jour (but not as many great street musicians here as in Nashville).

You will see naked people and they police probably won't arrest them if they aren't participating in lewd conduct. In fact, a state judge just told the city they had to let a "nude in" happen here last month.

Like Sophia said as long as you aren't hurting anyone most people won't care what you're doing...but picking on people, showing a gun, being a bigot or bullying "others" like routinely occurs in small towns all across America is generally not tolerated here. Many people have moved here because they feel like they don't "fit in" in other places in America where they've often had bad experiences and they are sensitive and will respond en masse if they see other people getting picked on.

The food is incredible at many places. You can get almost anything you want to eat or drink. The closest pizza shop down the street from me serves all the usual plus Alligator and Kangaroo pizzas as well as the latest vegan whatever. Tons of places roasting their own coffee, brewing their own beer, making their own pastry with fillings from scratch, and so on... Lots of other cultures represented here - especially the food. If you want Japanese food just go to Japantown. Chinese food in Chinatown, Italian food in North Beach, etc. etc. we love the food here. 

It's not uncommon to see Gay, Lesbian, and Transgender people or couples together or showing public affection. Also not uncommon to see LGBT rainbow flags, BDSM equipment (whips/chains), drug accoutrement and adult toys right in the window displays of many stores in San Francisco. Many "store fronts" in SF are straight up pot clubs (I think you do need a state issued card to actually walk in but it's not my thing so I can't say for sure.)

We have a fully restored and functioning fleet of vintage street trains and cable cars. A lot of our busses are electric. Many people have tiny or electric cars or just take Uber everywhere and don't even own a car.

These things are all unlike any of the small towns I've visited in America.

San Francisco reminds me of New Orleans but without all the drinking all the time, Southern food and great music. It is also a port city so it has that energy: San Francisco has a wild and fun personality!

It works well for people like me who used to get bored easily in a small town and people who always like to see and try something new that they've never seen or experienced before. It's also good for people interested in working in or learning about technology, of course.

Whatever your "thing" is you can find some of it here in San Francisco as it has a little bit of literally everything. Bring your wallet (it's expensive) and have fun!
San Francisco is not particularly culture shock-inducing, except for people who have never seen a modern American city of medium to large size. 

San Francisco has it's own unique culture and personality, but it doesn't really have anything within it that you wouldn't find in other cities. 

That is to say, if "small town America" were sincerely shocked by the experience of travelling to SF, then it stands to reason they would be similarly shocked by Los Angeles, Seattle, Las Vegas, Chicago, Cleveland, Boston, Philadelphia, Baltimore or New York (particularly New York actually). 

Having said that, people of all geographic origins tend to react negatively to the strong urine odor that many neighborhoods have, and the frequency with which human excrement can be seen on the streets, as well as the sheer number of homeless people.
It can be, for a number of reasons.

First, there's the pot. There's just so much of it. It's everywhere. Technically, it's illegal still, but the cops have bigger problems (Oakland, primarily) to deal with.

Second, there's the expense. San Francisco is not a cheap place. Everything is brutally expensive. Everything.

Third, it can be kinda rough. Oakland and the surrounding environs can be dangerous. There are gangs. There are many homeless.

Third, culture. As long as you're not hurting anyone, no one really cares what you do. Be gay, trans, be a furry, be a masker. It's whatever. In fact, if you raise voice or hand to a person for being these things, expect swift and severe retribution.
I once took someone from Wisconisn out to a big Chinese Dim Sum place in SF and he was freaked. out. He barely ate anything and looked completely bewildered. He was the only non-Asian person there. So then I was shocked by his freaked-outedness as I thought it would be a fun experience for him. This was in the '90s.
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