My kid says, "I got screwed." Now, should I take this as meaning someone literally fucked him up the ass, or can we agree this has become a figure of speech that transcends its origins?
I sympathize with the language police; it feels sometimes as if language is the problem and something that can be easily corrected. What they forget, however, is that it's the attitudes that drive the language, not the other way around. A perfectly decent egalitarian isn't going to turn into a misogynist by saying "cunt" or "bitch" or "pussy-whipped."
Language is not a static thing, though, we give it meaning and we take away meaning. We call someone a cunt not because it's a woman's reproductive organs and those are obviously bad, but because it's the Worst Thing You Can Say™. I daresay a woman's reproductive organs are rarely on anyone's mind when the term is used; it's the shock value alone that matters. Consider how "cunt" and "pussy" are both used pejoratively to refer to the same thing, but one is considered worse than the other.
All of the words we use have origins, and most are metaphorical in derivation. The reason we use cunt and asshole and dick and pussy and shithead and so on is because they are vulgar. Slate's Lexicon Valley is very helpful in understanding how profanity and vulgarity has changed. Look for episode #28. http://www.slate.com/articles/podcasts/l…
As for nigger and faggot, they're neither profane nor vulgar, nor do they have a meaning in US English that has transcended their origin (except when members of the target group have re-appropriated the term).