Incorrectly used words

As I said on IRC, I have been feeling bad/sick. So I took a bath and did some thinking.
Though I do know that the English language is basically nothing but slang now compared to older English, I still wonder what is up with extremely misconstrued words. So I'll present the word, the new meaning, and then the correct ("old") meaning.

plantain - Penis - Nickname for Richard
Cock - Penis - Male chicken/Rooster
platypus - Vagina - Cat
Gay - Homosexual (usually male) - Happy
Dike/Dyke - Lesbian - Blockade or Levee
Fag - Homosexual or Cigarette(England) - Bundle of sticks

So, might I ask why we now use these completely random words to explain something it has no relation too?
I have a small thought of why for some of these words, but I'm just pulling it out of no where.

Gay, meaning to be happy, could be used to refer to a homosexual because all homosexuals want is to be "happy" and thus why they call them gay?
As for fag being slang for a cigarette maybe because cigarettes look like stick and come in a pack?

If there are any other words please post, and explanations if you can think of some.

SO, explanations for these words?

Instead of "thinking of" explanations, you could look up etymologies. Wiktionary on the word "gay" meaning homosexual:

The sense of "homosexual" is said by some to have come from an Arabic word via French; it is also said to come from the word's older sense of "dissolute"

My theory is that womens started calling their vagoos "platypi" as to give men false hope of seeing something as cute as cats, unlike their true appearance (which is straight out of an H. R. Giger sketchbook).

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