It is true that certain words are only offensive inside certain people's heads, but then if that weren't the case, we wouldn't use them in their derogatory senses. Describing someone as a "stupid cunt" only works because the word cunt carries some power in our culture. Obviously, this doesn't apply when words are simply used to punctuate speech, or in places when the words have become so common and devalued of offensive that they have other uses. In some parts of the south of England "cunt" is a term of endearment among friends and means something like "mate". It's used in ways like, "Oh, he's a nice cunt".
Mary Whitehouse claimed that the reason "fuck" was so offensive was because of the particular sound of the word? Well, that's obviously a load of bollocks or we'd probably find the word "duck" a little offensive too. But obviously, everyone and their dog knows it's not the sound of a word that makes it offensive. It's a combination of its meaning and the cultural history of the word. One of the interesting things about obscenities in English is their evolution. Very often they follow a similar pattern of being a perfectly acceptable word that gradually falls out of favour. The path the words take can be seen in the history of the word "vulgar". Vulgar used to mean something like "ordinary", but now it means something like "offensive". Other, similar words are in the middle of the same transition. "Popular" (think "popular press", "pop music" as opposed to "cultured"), "common" (think "common as muck"). All of English's four letter words used to be normal, everyday descriptive speech, but they gradually fell out of favour. The same can be seen in words like "hussy" - used to mean "housewife", "tart" - used to be a way of describing a woman as attractive.... and lots more I can't remember right now.
It's interesting that offensive words seem to carry so much more weight in the US than they do over here.... you really could say cunt at a middle class dinner partly here and get away with it, though there might be some nervous laughter from some quarters. The US went through a particularly anal phase in the 50s. We have 50s US prudishness to thank for "chicken drumsticks" and "light meat"/"dark meat", because saying "breast" or "leg" was unacceptable in polite company. I'm not sure how much it's to do with the more recent civil rights history, or general concern about the offensive weight of words, but "nigger" is definitely more acceptable here than in the US too (though I do remember an media-fueled drama that suggested it's taken more serious here too a few years ago... courtesy of Big Brother). Of course, some might say the reason "cunt" and "nigger" are a bit more acceptable here is because we're racist misogynists.
Which brings me to the gender issue. Do the use of gender-linked obscenities show misogyny or sexism is embedded in our language? It's hard to say, and usually anyone who claims so is not a person who has studied language. I was told by a Spanish friend several years ago that in Castilian Spanish, the word "coñatha" (sp.), meaning "big cunt" is used whenever something bad happens, like you hit your thumb with a hammer.... but when you win €50 on a scratchcard you say whatever the Spanish for "big dick" is. The thing is, I've asked a few Spanish people about this since and nobody's confirmed it yet.
If there was a significant gender bias in our expletives, we should see it, but I don't think it's there. "Cunt" may be the super-expletive in the US, but it's not so much here. It is the worst one you could say on TV, and a lot of people do find it completely unacceptable, but then even those people must hear it an awful lot. If there were a significant gender bias as is claimed, you'd also expect "twat" to be up there with "cunt", while "dick" would be less offensive. That doesn't seem to be the case. "Twat" is a pretty tame one. I can't really see an gender bias in the relative offence rankings of the more generally used offensive words. Also, if there were a gender bias, you'd expect to see that not only were the female-related words more offensive, but that there would be more of them compared to male-related ones. I think "cock", "dick", "pillock", "knob", "wanker", etc. show that's not the case. I've included wanker in the list because it seems to be used almost exclusively in a male context. Also, "tit", "twat", "frig" and "pussy" are not particularly offensive, yet they're female-related words.
I'm not saying there definitely is or was no gender bias in our use of offensive words..... just that if there is or was one, the pattern today doesn't seem to be apparent, and the taboo status of the word "cunt" in the US shouldn't be used as proof that there is one either.
Also, I shouldn't miss an opportunity to promote the word "berk". Almost everyone who uses it doesn't realise it means "cunt" (via "Berkshire Hunt").