Marriage and long-term relationships are on the decline throughout the world as is marital sex, which has been reduced, in the United States, to about once a week for seven minutes at a time (which includes the time he spends begging).Why is marriage dying and sex evaporating? Because it is based today on the Christian concept of love rather than the Jewish concept of lust.The New Testament is very uncomfortable with lust. As St. Paul said, “Now to the unmarried and the widows I say: It is good for them to stay unmarried, as I do. But if they cannot control themselves, they should marry, for it is better to marry than to burn with passion” (1 Corinthians 7).Love, by contrast, was seen as lofty. St. Paul famously argued that “God is love” and that all marriages should be based on the comforts of compatibility, friendship, and shared experience. He regularly extolled the virtues of love in contrast to lust: “Love is patient, love is kind. It does not envy, it does not boast, it is not proud... it is not easily angered, it keeps no record of wrongs... Love never fails” (1 Corinthians 13).Judaism, however, believes that marriage must be built on deep desire and covetousness. The holiest book of the Bible, the Song of Solomon, is an erotic lust poem that describes the burning yearning between a man and a woman: “Your breasts are like two fawns, like twin fawns of a gazelle that browse among the lilies... Your stature is like that of the palm, and your breasts like clusters of fruit. I said, ‘I will climb the palm tree; I will take hold of its fruit.’” For us, lust is hot, sexy, and holy.The tenth commandment is clear: “Thou shalt not covet thy neighbor’s wife.” Which means, by direct implication, you ought to be coveting your own. About 80 percent of husbands who cheat on their wives claim to love their wives. So why are they unfaithful? Because lust for another woman has trumped that love. Lust is stronger than love. So why aren’t we using this powerful tool in our marital arsenal? And this is true for women as well as men. Look at the phenomenal success of the Fifty Shades of Grey trilogy.Why are liberated, educated women reading a book about a woman named Anastasia who voluntarily submits to being a “dominant” billionaire’s “submissive”? Because the essence of the novel is a man who lusts after a woman so mightily that he wants her more than anything. And for the average married woman who feels loved but not lusted after, the novel became a form of wish-fulfillment.How do we recapture erotic lust? By focusing on its three laws.THE FIRST is unavailability. Whereas love thrives on accessibility and constant companionship, lust flourishes on precisely the opposite: frustrated desire and erotic obstacles. Lust is enhanced through an inability to attain the object of one’s longing. It’s the reason that the Torah makes a wife sexually unavailable to her husband for 12 days out of every month (laws of Niddah), so that sexual hunger may increase. But it’s also true of every other area of life. The fare in every fastfood restaurant always tastes bad. The reason: nobody made you wait for it. Your taste buds had no time to salivate. There was no mental anticipation as to how the food would taste. But in an upscale restaurant they purposely delay your food, even if you ordered the ready-made special of the day. And why? Because appetite is enhanced through denial.The second law of lust is mystery. Lust is enhanced in darkness and shadows. Ironically, the more the body is covered the more one lusts after it. Modesty is not prudish. It is hot and sexy. The most boring place on earth is a nudist colony (or, er... so my friends tell me).It leaves nothing to the imagination. When a wife once came to me for advice on how to entice her newly-wed husband into having more sex, I told her to undress in the bathroom rather than in the bedroom. She thought the advice puritanical. I responded, “Disregard it if you wish. But you may face the nightmare scenario. That where you and your husband are married for four years.You come into the bedroom. He is watching television.You take all your clothes off... And he continues to watch television.”And the third law of erotic lust is sinfulness. You’re walking along a beach. You see beautiful women in bikinis. Is that sexy? Perhaps. Is it erotic? Definitely not. What do most men do at a beach? Either fall asleep or play Frisbee.But now you’re walking home. A woman has accidentally left the blinds to her bedroom open and she’s walking around in her undergarments. Same amount of clothing as a beach. Same amount of flesh exposed.Except this time it’s not a bathing suit, it’s her undergarments.What’s the first thing that comes to mind? I’m tired and want to sleep? Where’s my Frisbee? Of course not. So why is the second scenario so much more exciting and erotic? Peering into the privacy of a woman’s bedroom is forbidden. As the Talmud says, “Stolen waters are sweet.” Now you know why the Torah made a wife sexually forbidden to her husband for a portion of every month, thereby injecting erotic “sinfulness” into marriage.The “love marriage” is based on closeness and constant intimacy. The “lust marriage” is based on separation, renewal, and a measure of distance.By making lust kosher again we might just save marriage from terminal decline.The author, whom Newsweek and The Washington Post call “the most famous Rabbi in America,” will be publishing his newest book, Kosher Lust: Love is Not the Answer (Gefen), on May 1. His website is www.shmuley.com. Follow him on Twitter @RabbiShmuley.