Anon, are you familiar with the term ‘sodomy?’
It may explain a bit.
That said, if you consider the way that human beings actually use swear words, they often completely forgo any particular relation to the origin of the word. It’s a means of expressing distaste or insulting someone, yes, but it’s also a means of venting, much as the way someone stubs their toe and says 'ow’. Ow has no intrinsic value or meaning, it is a sound, a sound that your body and brain produces to distract you from the pain; swearing or screaming is a way of getting rid of or numbing yourself to pain, irritation or anger.
In other words, in the unique case of oaths and curses, the importance of knowing the original meaning of the word is lessened, because the function of swearing is not literal communication but emotive communication. One of the most interesting things about swear words (in my opinion) is that they are so versatile that you can not only use them as adjectives, nouns and pronouns, but you can also use them positively or negatively as freely as you wish. The meaning of swear words lies in context and expression. It is the intent of the speaker or writer that defines the word, not the original meaning of the word.
After all, when was the last time that you said 'motherfucker’ and genuinely meant what you said?