I didn't downvote the question, but it's not hard for me to guess what might be going on. The question seems frivolous, and it's not well-researched.
Okay, you've managed to notice that two vulgar slang references to genitals happen to mirror the name (or informal name) of an animal. So what? Two instances hardly a pattern make, so it seems a bit premature to ask about why such terms are "so common" (as I mentioned in my comment, I think #3 is unrelated to the bird, so that one could probably be struck from your list of examples). Moreover, how many other things (besides "dirty" names for body parts) use animal names? What about things that lead us astray, like red herrings, rabbit trails, wild goose chases, and barking up the wrong tree? Why do we talk about crocadile tears and the elephant in the room?
According to your standard, those might be "perfectly useful legitimate questions which are clear and answerable," too, but I wouldn't be surprised if they got downvoted, particularly if I asked about them with little mention or evidence of prior research.
I also believe Kate's answer went to straight to the heart of the matter: with so many slang names for genitals, it might be considered even more remarkable if none of them alluded to an animal.