I did explicitly say that people could use insults without knowing what the root of the word was. You've not paid attention to the entire thrust of my argument, that such an insult is not necessarily an indicator of prejudice in an specific individual, but its original basis and popularisation are based on some people's attitudes. The simple fact is that insults are rooted in terms that people believe (or believed, as they are often historical) have negative associations.
Next, more strictly, I said references to genitalia were crude. It's a little tedious of you to bait me, but if it'll clear up the cheap mockery, the same applies to "dick" as much to "twat".
Besides, the words are still unpleasant to some degree. Less so nowadays but much more historically, polite society avoided mention of genitalia and either waste extraction or reproductive functions of them, hence all those pompously delicate names for toilets (little boys room, powder room, water closet, etc.) It also ties in with old-style societal morals having distaste for sexuality, and the dirtiness of urinating and defecating. Calling someone by these sorts of terms was and is designed to associate them with those negatively-perceived functions - for instance consider that saying someone "thinks with their dick" is manifestly not a compliment, much as sex is enjoyable and some people like to have screwed a lot of others.