This is a two-parter: today, the evolution of offense; tomorrow (or eventually), how much our speech should defer to others' feelings.
I find the evolution of insults rather fascinating. Not so much the evolution from "misbegotten half-faced hedge-pig" (purportedly one of the ever-popular Shakespearean insults) to "you jerk," but the evolution of the meanings of "fag" or "moron" or whatever throughout time.
As is commonly known, "fag" (also meaning "cigarette") comes from "faggot" which at one time referred to a bundle of sticks or wood. Less commonly known (at least to me, until Wikipedia came along to enlighten me), "faggot" in reference to people likely originated as a term for the poor old women, usually widows, who gathered such bundles to sell as firewood. (The general habit of denigrating gay or effeminate men with terms originally used to describe women is particularly offensive in my mind, now from both directions though it was likely only intended to be offensive from the point of view of the man so impugned.)
In addition to slandering men by insinuating they might be as bad as—the horror!—a woman, slandering people by association with the less mentally gifted members of society is a perennial favorite. I find this evolution to be particularly interesting as it seems the medical establishment is constantly playing a game of linguistic whack-a-mole to replace terms that have been coopted as insults with more politically correct, less loaded terms...which are themselves coopted in short order. (Though, to be fair, it seems the medical establishment was not selecting labels with neutral connotations to begin with, as most of their terms have been in use in various derogatory senses for hundreds of years.) "Moron," "imbecile," and "idiot," though coming into the English language at different times, shared a period of time as part of a classification scheme for the mentally deficient (in order of increasing deficiency). "Cretin" at least started out with some degree of sympathy enshrined in its usage; literally it means "Christian," reminding one that this poor creature crippled by a thyroid disorder and low mental capacity was still, after all, "human despite physical deformities." So after every term already in use for the mentally handicapped becomes corrupted, on comes "retarded." As a term, I'm sure it originally seemed rather nice, implying simple delay or a slower pace. Of course, we all know how that worked out. Then there are the terms derived from the mentally ill segment of society: lunatic, deranged, crazy, insane, mental.
Gendered insults are fascinating. Basically any gender-specific body part is up for grabs, though often used in a less tidily gendered fashion. (This is where I get completely subjective and start going entirely off my own connotations.) Though of late there is more cross-pollination, in general I think terms dealing with male genitalia are only applicable to men (or at least started out that way): dick, cock, prick, jerk (well, the etymology of that one is slightly less clear), cocksucker (in an interesting turn of events, apparently cock-sucking isn't offensive to a woman who does it—nice to be given a pass for once). "Asshole" seems also to be primarily applied to men, though it's getting more equal opportunity these days. Women's anatomy seems up for grabs, though; "cunt" and "twat" seem like they were used probably more to describe women, though "cunt" at least is shifting man-ward. "Pussy" has, I'm pretty sure, always been directed at men. (Because, you see, women can't be expected to be courageous, so by a simultaneous usage of analogy and synecdoche, wimpy men are pussies. I was about to use "wussy" instead of "wimpy," but then the internet informs me the two likely etymologies for "wuss" are "pussy wussy" (as referring to a cat) or a portmanteau of "wimp" and "pussy." Well, there we go again.)
Then there are the blatantly misogynistic terms, all applied to women, of course (bitch, whore, slut—and yes, I know, "bitch" is becoming equally applicable these days, but when it's used for guys, it generally seems to be taking all the negative female aspects of the term and applying it to the guy for a double whammy of insultingness: a female bitch is mean or crabby or overly assertive; a male bitch is whiny or wussy or whipped, or he's someone's bitch, a loss of power not really implied with the female version). "Bastard" and "son of a bitch," though rarely used in their literal senses anymore, in those senses would deprecate the woman/mother more than the man being so designated. ("Motherfucker," however, would actually seem to only impugn the man...of course, dare I suggest it's because the mother is either already assumed to be depraved or because she is obviously powerless and without agency?)
And now, to my favorite (in some strange sense of the term): "douche" or "douchebag." Though obviously the intent is to insult a man by calling him something associated with the vilest of vile, the very dross of a woman, surprisingly, I have absolutely no (gender-associated, feminist) problem with this one (the word in general I find grating, but that's a different issue). Firstly, the implied action just doesn't have the same kind of weight. (What, you're so gross/insignificant/disgusting that you could be the thing to clean a woman's vagina!? Doesn't seem quite so intuitive as most of the rest of them. (And wouldn't a clean vagina be seen as a positive thing by the sorts of people who start using these words, anyway?)) But best of all, (though this is an appeal to modern thought, so it goes slightly against my general literal, historical, and semantic arguments elsewhere) douches are no longer seen as a good thing! Women (generally) accept their vaginas as healthy and clean the way they are. Douches are decried by doctors and even women's magazines as a good way to upset one's natural balance and cause infections and all sorts of lovely things. Modern enlightened women find them repulsive and more than a little oppressive (especially when taken in tandem with the past habit of using Lysol as a douching agent—yeowch). So if you want to insult a man with a term related to a woman, "douche" is your word! It's not offensive to women exactly because douches are themselves offensive to women, so who could argue with applying their label to people who offend? Or that's my take, anyway, and apparently one in which I'm not alone.
Some day I would be interested in being able to compare people's definitions of jerks, bitches, assholes, douches, etc. In my estimation it's clear they aren't simply used to fill in the blank when an insult is desired; they each apply to distinct groups of people. I imagine one man's "douche" is another man's "asshole," though.