"Sed a... said a dirty word, 'e said a dirty word!" - Igor, from Young Frankenstein.
Sorry, s'what popped into my head. Cuss words seem to be completely arbitrary. It's culturally defined, so what is a bad word in one context (you wouldn't say fsck at a job interview) is more or less fine in another (you might say it with your friends). It all depends on the image of yourself you want to portray, I guess.
Perhaps the severity of the words comes from the thing they represent? If you saw some gal's t!ts, you'd be less shocked than if you saw two people fscking. Make sense?