That is a good question; I don't know how over time it has become more obscene, but maybe it is because when the 'c' word is said to a male, it is more of an insult, because it is suggested they have female genitalia rather than male? At least if the 'd' word is used, it is not threatening their masculinity? Just a theory. As for the reverse, I have never heard the 'd' word to be used towards a woman.
But also the real story go's back to England, and is far too long for here.
As for the C word, it was a word for a woman's most private area, of her body.
If said today, we feel it just is too raw.Anyway it is a ugly word indeed.
I find it very dumb when one refers to another as D or C or anything at all inclusive of one indulging in the act of copulation!
It's mindless any way you look at it. Whether D or C or D-head. People should be taught to use proper words when one refers to a person dimmer than themselves or just for the heck of mocking another.
How does using a body part name imply mocking/heckling another and if the parts do imply anything like that, can one live without the part?
My Dad usually says "Useless fellows!" That's powerful enough to replace the use of such words!
Maybe because it's the female of the species' who gives birth to the next generation, so any 'obscene' word that can be used towards that part of a female's anatomy is considered 'having a go' at a mother. You can't get more personal than that.
And as it's usually uttered by males', deep down jealousy perhaps?
I think because of the connotation of the "c" word, and the fact that the word itself is ugly. Whereas the "d" word is more of a humorous expression to an extent although, strangely, applying to male genitalia.
Why it should be humorous on one usage and ugly on the other is odd really.