Everybody learns that in -- what? -- fourth grade? After that, the subject is not so simple. It is, for certain, one of our species' favorite subjects to think about, much less do. (It's how we all got here after all, with the exception of rare test-tube babies and cloned barn animals.)
We have a distinctly American way of presenting sex, though, which is, these days, inextricably bound up with health. It's the genius of American pop culture that the way to sell sex onscreen is to pretend to put the focus on something else and count on our species' natural prurience to do the rest.
It was that Hollywood grandee Cecil B. DeMille who took his cue from grand opera and confirmed, for generations of American filmmakers, that you could ensnare millions of hungry eyes with scantily clad debauchery while purporting to present religious allegory.
Whether Mel Gibson's "The Passion of the Christ" transfers the DeMille Method to violence is a subject we can all debate until the cloned cows come home.
For now, though, consider the new way we do sex -- the alliance of sex and vigorous physical health, and whatever else can be thrown into the center of the subject to disguise the fact that what's really being presented to us is comely flesh.
If fitness is the new American religion, we have, once again, married sex to it -- this time, though, promised as the end result of eating right and working out rather than the old idea of the thing one must repent through piety and good works.
Take the hugely popular "Dancing With the Stars," the greatest thing to happen to reality TV since the reawakened "American Idol." Here is serious prime-time ammunition to use against the Olympics.
Everything -- and I do mean everything -- about "Dancing With the Stars" is perfectly designed. Host Tom Bergeron has been one of the wittiest and glibbest freelance mouths on television for years. The idea of home viewers passing instant judgment on what they've seen is never going to outgrow its appeal. And the three judges a la "American Idol" are perfectly chosen: one strict stickler who happens to be beautiful, one benign but candid paternal type with a perpetual cockney wink and one-hot blooded exaggerator who lives to emote on camera and overstate the obvious.
All of which yanks the ostensible focus of the show away from its true one: the sexiest costumes and the most shameless bumping, grinding, shimmying and slithering we've ever seen in a weekly prime-time show.
What is causing American jaws to drop from Baja to Bangor are sensational looking women dressed with as much artful skimpiness as possible and doing pelvic thrusts that might strain Olympic athletes.
They can claim on camera what got us all into the tent was the chance to evaluate someone's samba or pasodoble but what we're watching is the rawest version of Little Egypt and her hoochie coochie dance we've ever seen on prime time. If June Taylor had come up with some of this choreography for her dancers on a Jackie Gleason Show a half century ago, Gleason would have immediately put her on the first bus to Passaic with a one-way ticket.
But it all comes to us with weekly stories -- can Drew Lachey maintain his streak? Can Stacy Keibler, the professional wrestler (no kidding) with the mile-long legs keep on wow-ing one and all? Will hot, hot Lisa Rinna justify all those collagen injections? Can George Hamilton charm and clown his way through another week? And it's all presented in the context of the fiendish physical self-discipline that dancers really have to have. Add it to physical health beyond many, if not most, athletes and what Puritan could possibly disapprove of an hour of so much physical well-being giving proof of being elect? Seldom is the Triumph of Belief so impressive.
It leaves the scramblers and backstabbers of "Survivor" -- the grandpa of Reality TV -- in the dirt, just as it does the soap opera dramatic pizzicato of "Desperate Housewives."
You don't see prime-time junk perfection often. Enjoy.