I have not owned a car since 1988. But in the early 1980s, I had a bumper sticker on that car that read, “Nixon/Manson in '84.”
“Alcohol and Calculus don’t mix - never drink and derive”
On my RX-7, I had a license plate frame that said “If you can’t stop, wave as you go under”. Ideal for this F-350-type neighborhood.
I have a “Wall Drug” sticker just waiting for a car to put it on.
An old, faded Human Rights Campaign sticker. I used to have a Darwin fish, but someone removed it.
I have a “Seahawks” bumper sticker. I once had one that said “My other vehicle is a TARDIS”, but that car is long gone.
I have BDA (Bermuda), W (for Wellfleet, MA), MTK (Montauk, NY) place stickers. I also have a Bernie Sanders and a Hillary Clinton sticker.
On my first car in grad school I had “Pro-choice is Pro-family” sticker which was routinely vandalized.
^ SXM (St Martin)
I used to have a “Stupid People Shouldn’t Breed” bumper sticker. The people who understood it laughed; those who were puzzled by it, well.
“Driftless.” It’s a shout-out to my newly adopted home region’s geomorphology. I’m sure it leaves many (even around southwest Wisconsin) scratching their heads – maybe they think I’m leading some kind of lifestyle or practicing some philosophy: “Aimless drifter? No, maybe the opposite of that…”
How could this be puzzling? Honest question.
Love it!
IMHO. 
You got me. I’d see people looking at it as if they were trying to sound out the big words.
Perhaps some folks see the division between “humans” and “(other) animals” as so strong, that they have trouble parsing how the verb “breed” could apply to humans?
That was my thought immediately upon reading the thread title. Been some years since I’ve seen cars with conventional bumpers. It did make me think of when I had a bumper sticker on my '65 ford Galaxie 2dr. Hard top. Dual Chrome pipes out the back. Was crossing the border from saskatchewan into Montana in 1973 at the height of Watergate. The US border guards wouldn’t let me cross until I scraped the sticker off. It said “Dick Nixon before Nixon dicks you”. They were less than impressed. 
That kind of seems like an overreach of authority on their part. :dubious:
My wife had a '92 Ford Escort she finally junked three years ago. One day, I stepped on the “bumper” to fish something out of a dumpster. I left a foot-sized hole in the plastic. :eek:
Back in the late '60s/early '70s, Rock City (A tourist attraction on Lookout Mountain in Chattanooga, TN) employees used to slap their bumper sticker on your car while you were enjoying the trails. They must have caught some guff for it; a few years later we went again and when we came out, there was a heavy cardboard “bumper sticker” held on with metal straps tied onto the bumper. It hung in the club basement for many years.
I have a Dan’s Comp, Pull BMX, JR Bicycle, Ohio Mountain Biker, GT, and Haro Bikes. I also have a Chelsea Football sticker. It’s pretty much a rolling billboard for bikes and soccer…
Oh, man, I’m looking at decals to replace the old ones on my car, and there are too many choices. I can’t decide between ‘‘Slug Life’’ (with slugs), Futurama brain slugs, nudibranchs, Cthulhu Nom, or ‘‘I love snails.’’ There are also a variety of jellyfish and other tentacled creatures. It’s just too much to handle.
Didn’t help that I was wearing an embroidered jean jacket and near shoulder length hair. I think they thought I was a potential drug peddler (which I wasn’t of course).
I’m in a state where I don’t need to have a front licence plate, so I have this instead:
I’m sure biologists will recognize it, it’s this:
#/media/File:Darwin_Tree_1837.png)