Neil the Ethical Werewolf has an excellent post about the connotations of sexual obscenities. I can relate to Neil's tendency to analyze foul language.
Swearing is a fascinating empirical and philosophical topic. Why is it that people reflexively scream taboo words in non-sensical contexts when they're angry, scared, astonished, or otherwise riled up?
One interesting thing about "swear words" is that they're conventionalized. You can't paraphrase expletives, except by substituting a culturally approved euphemism. For example, you can't use "sexual congress" in place of "fuck" in real swearing situations. "Screw you!" will work as a substitute for "Fuck you!", but "Get laid!" just won't get your point across.
Only certain words can be used to telegraph that kind of raw emotion. Kids are very interested in figuring out exactly which words are on the list. Do you remember intense debates on the playground whether some racy word was "a swear" or not?
Pure expletives seem to work independently of the nominal cognitive content of the expression. When someone bangs their thumb with a hammer and yells, "Fuck!", chances are that sexual intercourse is the furthest thing from their mind.
It's surely not coincidental that our culture's forbidden words usually have to do with sex and bodily functions. In Quebec, where the Catholic Church has historically cast a long shadow over daily life, the offensive expressions tend to be blasphemous rather sexual or scatological.
Interestingly, Quebec's atheists and agnostics use these blasphemous epithets with just as much gusto as believers. "Sacre!" still works, even if you aren't Catholic and couldn't care less about the church. English has a few blasphemous expressions, but "damn it" just doesn't pack the punch of "fuck it." "Bloody" and "hell" are hardly even rude anymore. ("Bloody" is a reference to the blood of Christ.)
Gutter insults are a little different from pure expletives. Most taboo names are actually similes. You can insult someone by directly accusing them of being stupid, lazy, inept, sexually unattractive, dishonest, or whatever. Or, you can use a curseword that stands in for the particular set of bad qualities you want to attribute to someone.
If you call someone "a pussy" you're imputing stereotypically feminine faults to them: namely being weak, squeamish, and cowardly. If you call someone "a prick" or a "dickhead" you're delivering a very different insult. By calling someone "a whore" you're attributing characteristics that our society normally associates with prostitutes: either a willingness to debase oneself for money, or sexual promiscuity. As Neil, Amanda, and zuzu observe, that elision only works because it piggybacks on our society's contempt for sex workers and sexually active women in general.
By contrast, there's no difference in the meaning of "Fuck!" vs. "Shit!" as a pure expletive, even though the two words refer to totally different things.
Some abusive taboo words function more as similes than others. "Bastard" used to be tied to the stigma of illegitimacy, but not any more. I remember being puzzled as a child when I found out that "bastard" literally meant someone whose parents weren't married when the were born. "Why would real bastards be more likely to be backstabbing or obstreperous?" I wondered. "Idiot" used to be a direct allusion to mental retardation. Nowadays, most people consider it an interesting bit of trivia that the term was originally a legitimate medical classification for developmental disability. To most people, "idiotic" is just another synonym for stupid.
Maybe taboo words have a life-cycle. They start out as ordinary words for taboo things. For whatever reason, some of them get picked up and conventionalized as expletives and/or terms of personal abuse. As language and norms change, the insulting connotation can remain long after the original taboo has eased.
It's interesting to contrast the status of the word "cunt" in British English vs. North American English. In the UK, the "cunt" seems to be going the way of "fuck" and "shit"--a general-purpose epithet that nobody particularly associates with female genitalia in the context of cursing. "Cunt" has even become a verb, as in "cunt off" and an adjective, as in "The cunting boiler is broken again." Whereas in the US and Canada, you can't use the word "cunt" without triggering associations with female genitals.
If taboo words have life-cycles, that might help explain why there are such vehement disagreements within the progressive community about the appropriateness of certain insults. Swear words reflect traditional values, that is, traditional in the sense of being widely held for a very long time. Our taboo vocabulary is a legacy of our actual taboos and hang ups.
When you use a word like "cunt" to mean a disgusting, immoral, or dissolute person, it's hard to escape the implication that cunts themselves are disgusting.
Notice that "dick," "prick," "dickhead," and "schmuck" are much milder insults than "cunt." What do you think that says about our society's attitudes, past or present, towards the female genitalia? However, you can instantly up the ante for male-genital-based insults if you add an implication of gayness, i.e., "cocksucker."
Most insults are somewhere between a live simile and a dead metaphor. People who speak the same language can disagree about where a particular insult falls on the continuum. At a certain point, "bastard" ceases to be a dig at a person's parentage and becomes the industrial-strength counterpart of "jerk." I don't hesitate to use the word "lame" to describe something inane, but I won't use the word "gimped" to indicate that something's broken. I don't use the word "cunt" as an insult, but I occasionally call wimps "pussies." For some reason, "gimped" and "cunt" just feel too closely tied to values that I reject. I don't have an argument for drawing the line exactly where I do.
Why are progressives, myself included, sometimes tempted to use insults that have sexist connotations? There's the lure of the forbidden, I suppose. There's also the desire to throw out the word you know to be the most offensive, literal denotation be damned. If you think that Michelle Malkin is the most contemptible person in the media, it's tempting to deploy the c-word simply because it's the rudest one-word thing you can call her. Some philosophers would argue that to you "cunt" just means the most despicable kind of person. (A lot of words we now consider to be homonyms started out with the same or similar meanings and drifted apart over the years.)
Most importantly, if you're going to insult someone, it's important that you pick an insult that will actually shock and hurt them. If you're trying to insult someone who's sexist and homophobic, it's probably more effective to call them "a pussy" than "a wimp." Maybe this is also the best reason to stay away from these kinds of words--they force you to play within your opponent's value system. As such, they are the consummate failure of framing.
Calling Michelle Malkin "a cunt" is the equivalent of calling the Republicans "the party of big government"--a terrible rhetorical move whether it's deserved or not. "Big government" is a Republican frame that Democrats have to counter with a better frame of their own. Fighting about who's really the party of big government just helps the Republicans by reinforcing their way of looking at taxes and the state. Likewise, "cunt" and "fag" originated in a conceptual scheme where vulvas are gross and gay people are subhuman. It's very difficult to use those insults without reinforcing the values that made the epithets make sense in the first place. An individual can sever the tie between the word "cunt" and cunt-hatred, but that doesn't mean that word has lost its associations for its audience.
Neil says that he doesn't use words like "cunt" very much because the negative connotation of the word is at odds with the positive associations he has with actual female genitalia. That's more or less how I feel about it. If a metaphor is live for you, and you disagree with what you take to be the underlying value judgment, it's less satisfying to use that phrase as a term of abuse.
[Mandatory context disclaimer: I'm not saying any word should be off-limits. Context is everything. Obviously, the standards are different for using a word in fiction, satire, direct quotation, etc. The appropriateness of the term also depends on who uses it, and for which audience.]
Amanda reports that online vandals in Second Life defaced John Edwards' virtual headquarters. 
