Like most legal questions, whether an offensive bumper sticker will be considered illegal depends on the details. Generally though, so long as the bumper sticker isn't obscene, or pornographic, or blocking your view, then in all likelihood, it's covered under the First Amendment's free speech protections.
Unfortunately, figuring out what is obscene isn’t as easy as figuring out what's offensive. And as a result, unbelievable news stories pop up about people (okay maybe just one guy from Florida) with their cars adorned with the phrase "I EAT A**" getting arrested for refusing to remove it. Yes, that really just happened to one Florida man who thought it'd be funny to adorn his pickup with a window decal sharing TMI with the world.
Being Offensive Is Not Illegal
While parents may rue the day their children see that window decal and have to answer the "what does that mean" question, the criminal charges against Dillon Webb were dropped at the first sign he was contesting it. Now, Webb is considering firing back with a lawsuit of his own. Webb was actually arrested after he refused an officer's order to remove the last S from his "I EAT A**" window decal.
This story isn’t so unusual, believe it or not. Not too long ago, big headlines were made after one man played a joke on his brother, and applied a crude depiction of stick figures having sex, along with the phrase "Making my family" underneath, to the back window of his brother's car. While that's a pretty funny joke between brothers, the younger brother got pulled over for it by the cops while driving in Tennessee. But just like in Webb's case, after lawyering up, things changed.
Curiously, while states may not be able to restrict the messages people share on their bumpers, even if it includes vulgar language, states can certainly restrict how that message is displayed in various ways. One common state law involves a ban on placing stickers on the windshield and rear window of the car. Typically, most states' laws will prohibit blocking more than a tiny portion of a car's windshield with stickers or decals.
Notably though, if you are considering putting an offensive message on your car, you might want to think twice as it may make it more likely you'll be pulled over for other reasons, or harassed by those who don't like that message, or believe it's inappropriate.