Recently, the online magazine Slate had a special issue devoted to sex. One of the articles, called Don't Ask the Sexperts featured prominent sexperts talking about what still mystifies them about sex. If our model holds any water, we should be able to answer some of their doubt and questions in terms of what we've learned about prehistoric sexuality. We'll post their questions and our responses periodically. Here's the first installment.

Rabbi Shmuley Boteach is the author of Kosher Sex, Kosher Adultery, and Shalom in the Home.

"What I don't understand about sex is the fundamental contradiction it poses to love. While love deepens with time and shared experience, sex is stifled by relationship and routine. It seems to thrive most through novelty rather than intimacy, through new flesh rather than old love. Sex is the quintessential expression of love. We even call it lovemaking. So, why are so many couples who are so deeply in love with each other, after so many years of being together, utterly sexless? Why must couples choose between being lovers and being best friends, between being passionate and being intimate? King Solomon proclaims in his famous Song that there is a love like fire and a love like water, and it seems that one cancels the other out. The fiery love of sex and erotic passion becomes more and more muted through the more watery love of marital routine, familial patterns, and an increasing number of anniversaries celebrated. Why can't sex and love go absolutely hand in hand, as they should?"

It always amazes us when people use the word "should" in talking about sexuality, as if we choose what to find erotically stimulating. Do we choose what smells good or what colors soothe our eye? Did you "choose" to love Mahler's 2nd symphony or Exile on Mainstreet because someone told you that you should? We're not so sure that "sex is the quintessential expression of love." Surely, sex can be a profound expression of love, but there are so many ways to express love, not a few of them being far from erotic. Think about the love expressed in caring for a disabled spouse for example. Or the love that goes into changing a baby's diapers day after day. You get the idea. Love's not always about sex.

Again, this is not to say that sex cannot be an extreme expression of love. It can. But sexual novelty is too important a part of erotic attraction for most men to be completely comfortable with simply stating that sex is all about love, period. Sometimes it is; sometimes it isn't.

Rabbi Boteach and others may be interested in the Coolidge Effect, so named because of the following story: President Coolidge and his wife were touring a large chicken farm. Because the president had to attend some meetings, Mrs. Coolidge took the tour first. Upon entering a large area with hundreds of female chickens but only one rooster, the First Lady commented on the imbalanced ratio. The tour-guide explained that males would tend to fight if placed together and that one male was enough in any case. “Really,” she asked, “how many times does that male mate with females?” “Oh, dozens of times per day,” she was told. “Well, when my husband comes through here, be sure to tell him that” she replied. Later, on his pass through the same room, the president was told of this exchange. “Interesting,” he said. “And does the male mate with the same female each time?” “Oh no, Mr. President, with a different female each time.” “I see,” he said, “would you please tell that to my wife?” Hence, the Coolidge Effect.

While the Rabbi and others might find the Coolidge Effect to be a source of great sadness, and let's face it, it is a source of great sadness and dissatisfaction for many couples, there is nonetheless very strong biologic (to coin a phrase) underlying the primarily male desire for erotic novelty in many mammals (human included).

Donald Symons, in the now classic The Evolution of Human Sexuality, put it memorably by noting that, "Human males appear to be so constituted that they resist learning not to desire variety despite impediments such as Christianity and the doctrine of sin; Judaism and the doctrine of mensch; social science and the doctrine of repressed homosexuality and psychosexual immaturity; evolutionary theories of monogamous pair-bonding; cultural and legal traditions that support and glorify monogamy; the fact that the desire for variety is virtually impossible to satisfy ...." He goes on, but you get the point: Life would be so much easier for men if they weren't like this, so we can be pretty confident that this isn't a hankering anyone is actively choosing.

(The parallel to homosexuality is obvious. With homophobia still so pronounced in most parts of the world, it's a mystery how can anyone seriously think millions of people are simply deciding to be gay.)

Without the Coolige Effect urging them toward new sexual partners, the small, isolated bands of hunter/gatherers that comprised our species until relatively recently would have simply in-bred themselves right out of existence. None of us would be here now, wondering why familiarity numbs eroticism. Hybrid vigor is as important in people as it is in tomatoes, so men have evolved to be turned on by the unfamiliar and sexually numbed by sustained familiarity.

Whether or not we think it "should" be so, so it is.

Turns out, the incest taboo has deep biological roots. Married couples often do start feeling like siblings after a while. How can they not, living side by side day after day, night after night? Masters and Johnson and many others have noted that many men complained that sexual monogamy could start to feel a bit incestuous after a while.

Sexual attraction is about drawing two separate beings together. Once they are together, the function of this desire has been fulfilled

Perhaps an understanding of why this happens may help couples move beyond blame, shame, and guilt and begin thinking about how to deal with this extremely common predicament constructively.

After all, let's remember that sex itself is not a biological necessity: many species do just fine without it, relying on parthenogenesis instead (virgin birth, essentially). The central purpose of sexual reproduction is to keep the salad tossing and the genes mixing -- not, as Rabbi Boteach and many others wish, to celebrate or promote love. Love is something that got added to the process long after the process itself came into being. There can be no doubt it's a wonderful addition, but love is certainly not an integral nor a necessary part of sexual reproduction.

So, with an understanding of human sexuality informed by this knowledge, what might we expect to find in long term sexually monogamous heterosexual relationships?

We'd expect to find that males were eager for sex with their mates in the first few years, becoming steadily less so as time passed. They'd be likely to feel confused and shamed by these feelings, not understanding how they could feel their sexual attraction diminishing even as love and intimacy deepened (even as their culture insisted that love and sex were part of the same complex).

Sound familiar?

Oh yes it does.