who decides if a word/phrase is taboo e.g swear words?

[mad_for_it]

why are some words deemed to be offensive even if they are just slang for a medical/socially accepted term?

[geofbob]

I don't know which particular words you have in mind; but I don't think many words are thought to be offensive simply if they are used as slang for body-parts, like "boobs", "willy" or "bum". But, on the other hand, the c word is not usually employed simply as a substitute for a body-part, but in an extremely offensive way; and it's therefore not surprising if other people regard the word as taboo and avoid its use.