There is nothing nauurally obscene about holding out a finger, flipping the finger is a cultural thing:
And why is it called "the bird"?
It's interesrting, some folks say that flipping a finger has been an insult for over two thousand years!
"An Athenian comedian by the name of Aristophanes provided the earliest literary reference to the gesture – he created a feisty character who gave Socrates the finger."
In 1644, John Bulwer suggested the use of the middle finger for the deaf as a "natural expression of scorn and contempt."
Are obscene gestures illegal?
Yes!
In many states you can have people summoned to cpourt for flipping the bird!
It falls under "fighting" statutes, gestures that are incendiary, "fighting words" which are likely to escalate into road rage or assult.
I’m amazed how many citizens don’t understand thelimits to the protection of the First Amendment
