I think the answer is both. Vulgar words do get censored because of the concepts they refer to, but the concepts they refer to are not limited to their literal meaning. Other words with the same literal meaning are considered acceptable.
Indeed, sometimes the same word will be considered acceptable or unacceptable depending on whether it is being used merely for it's literal meaning - such as the word 'bastard' meaning a child born out of wedlock, or whether it is being used with an additional meaning, that of a person who is considered despicable in some way.
Such words are generally considered wrong because of their usage more than their actual meaning - and some words are considered wrong altogether because they are usually used without their literal meaning. To see why, consider that society finds it appropriate to have a discussion about the existence of paedophilia. But most would find it inappropriate to enthusiastically discuss that subject in every conversation. If you were having a casual conversation about football and felt it necessary to work the word 'paedophilia' into every sentence, people would find it disturbing because although they don't mind considering that subject in the proper context, they don't want it constantly brought to their attention. For years, most people felt the same way about faeces, reasonably enough, or sex. So naturally a lot of people find it unpleasant that words with those meanings are, by some people, used constantly and in public, when they aren't even talking about those subjects and have no reason to be mentioning them.