Sigmund Freud (in)famously opined that "anatomy is destiny", that (to oversimplify greatly) one's personality and one's potential were, to a large part, determined by what equipment one possessed between one's legs. Penis envy (he has one, and I don't!), castration anxiety (she doesn't have one, maybe they'll cut mine off, too!), and other Freudian concepts stem directly from whether you are an innie or an outie (so to speak).
The phrase has evolved a bit, and now is also seen as "biology is destiny", with somewhat fewer genital-related shades of meaning, but the earlier meaning is sometimes (often? I have not done a thorough review, so cannot say) lurking just under the surface. Whether our reasons are Freudian or Darwinian, there seems to be enough interest in that one set of complementary organs to support several industries... and the continuation of life as we know it.
We have long known that the brain includes multiple areas involved in face-detection; I begin to wonder if the entire rest of the brain might not be involved in genital-detection. We see them everywhere.
Take plants. I have a cousin, an artist, who (decades ago) exhibited a number of paintings of plants, and of close-ups of parts of plants. It probably won't surprise you to know that a split-open peach pit, in the proper perspective, will make the vast majority of a family gathering blush. It looks quite like the anatomical wall chart I once saw at an OB-GYN's office. Robert Heinlein's "Notebooks of Lazarus Long" includes a phrase that puzzled me when I first read it: "Have you noticed how much they look like orchids? Lovely!" And more recently, PZ Myers' "Wednesday Botanical" posts have included both phallic and yonic photos (or perhaps that is all in my perception).
Oh, underused powers
Of beautiful flowers;
They tantalize, tempt, and entice,
Whether insect or human,
When flowers are bloomin'
There's something that makes us look twice.
The curves I adore, kids,
I oft find in orchids
(Such flowers are dear to our hearts)
It's not quite the same in
A pistil or stamen
But sometimes, it seems, parts is parts.
In just the right lighting
It's rather exciting
When beautiful form follows function
In plant pollination
Or *our* fornication
When parts can perform in conjunction
That such an attraction
Creates a reaction
Is fact that a blind man could see
You might think me crazy--
I'm off to find Daisy
To ask if she'll just let me bee.