Just like mitochondria & cell nuclei, genitalia were once free-living organisms. They resembled hydrozoans: the penis a polyp & the vulva & vagina a medusa. They were, in other words, the same organism at different stages. Though it needed an anchorage, joined to its brother polyps by hollow roots, the primitive penis strained for more, combing the currents with a hungry cluster of tentacles. Eventually, a bud would sprout from its side, break free & open like an umbrella in all that water. So while the penis lived from hand to mouth, the vagina, arrayed in glory, neither toiled nor spun, & the music of its transparent bell spread through the oceans & inspired the coral to new architectural heights. And the god of evolution, swaying in the garden, saw that it was good.