It all started in 1853, when terms such as "grousle" "heckle" and "pettifogging Tammany Hall hucksters" were thrown about... People chuckled behind esteemed people's backs at the slight mention of grousle.
"Tehehe" - they all giggled.
Well now, who the f*** giggles when another says "f___ you"?
NO ONE.
So in 1854, a convention gathered in Oklahoma, and they set out to create new insults. Sex was not oft talked about, so they wanted to come up with a word that could thrown into any part of speech, and still sound good. I mean, you couldn't yell "You went and doggone that all up." Hence the term F___ came about.
It's really an elaborate history of swear words.
Also, sphincter is funnier than anus.