Love's mysteries in souls do grow,
But the body is his book. - John Donne (1572-1631), English preacher and poet
There's the Darwinian's answer--that if our ancestors hadn't been obsessed with sex, we wouldn't be here--which is reductive but incontrovertible. And then, there's the Romantic's answer--which, while ennobling, leaves plenty of room for argument. Rather than denude sex of its mystery and risk boring you, I'll take the latter approach and chance your disagreement.
Sex is mysterious, often sublimely so. Sex can open us to ridicule, or to enlightenment. Nothing about sex is for sure, except that it's consequential. The philosopher Arthur Schopenhauer writes:
The ultimate aim of all love affairs is actually more important than all other aims in life... . What is decided by it is nothing less than the composition of the next generation.
Schopenhauer, who lived a hundred years before the decoding of the genome, was not referring to its composition. Nor was he merely pointing to the demographic consequences of sex. He realized that sexual love goes beyond gene-swapping and reproduction; that our loves and their sexual consummation affect the composition of our psyches and our identities. A sine qua non of pro-creativity, sex is a wild card in our creativity.
I came across the Schopenhauer quote in a biography of Erwin Schrödinger, one of the creators of quantum theory, and a legendary lover of women. In his biography, Schrödinger: Life and Thought, Walter Moore breaks new ground by writing about both his subject's love life and his scientific life. He reveals that Schrödinger discovered quantum theory while hunkered down with a lover in a Swiss chalet. What makes this intriguing is that when Schrödinger was pressed to write about his creative life, he demurred, saying that he felt the part his lovers played in it was crucial, and discretion would require him to leave that out.
Sex changes hearts and minds like nothing else. It opens us to our partner and we absorb something of his or her ways, rhythms, and perspectives. Under the hex of sex, a nobody may become a somebody, or vice versa. Our habitual take on the world is temporarily suspended; the script we follow momentarily disabled. In the space opened by sex, we see freshly and may recognize something we've overlooked or spot something altogether new. Evidently, that's what happened to Schrödinger. And what was revealed to him, as the snow piled up around his Alpine love nest, were the secrets of the subatomic world.
Of course, not every lover opens us to world-changing or even life-altering discoveries. Some actual or potential partners are personal poison and it's important to cultivate the skill of recognizing who these are before leaping into bed with them. But, in focusing on seduction and mechanics, we're apt to lose sight of the unsurpassed power of sex to open us to transformation.
Even if there were no fear of unwanted pregnancy or sexually transmitted diseases, there's reason to be choosy about whom we sleep with. As diet affects the composition and well-being of our bodies, so sex changes the composition and integrity of our innermost selves. Is it any wonder we're obsessed with it? Not for nothing is this honey so sticky.
I've never been able to find the reference, but I've heard Sigmund Freud quoted as saying that sexual investigation and intellectual investigation go hand-in-hand. D. H. Lawrence, whose Lady Chatterley's Lover brought sex out of the closet, wrote that "The intelligence [that] arises out of sex and beauty is intuition." Jim Morrison cut to the chase with "C'mon, baby, light my fire."
Perhaps no one has ever shone a brighter light on eros than John Donne, the churchman, poet, and Don Juan quoted in the epigraph. He deserves the last word:
Who ever loves, if he do not propose
The right true end of love, he's one that goes
To sea for nothing but to make him sick.