Funny, Chuck, I was just thinking about this last night for some reason. Either I have telepathy and knew you were going to bring it up, or I just like to sit around contemplating terms for genitalia.
Anyway, what I decided is that many people are uncomfortable saying either "vagina" or "penis" in casual conversation. Sounds too ... seriously anatomical or something. So they come up with cutesy names for those parts.
When my boys were really little, some of their daycare providers called it "bobo" and others called it "peepee." My older son put them together. So we took to calling it "peepeebobo." At least then when they screamed it out in the grocery store, other shoppers didn't always know what they were talking about.
Of course now that they are teenagers, they have graduated to "wiener" but, thankfully, no longer scream it out in the grocery store. And they never refer to the other thing at all.