My friend Charles Krauthammer makes the argument succinctly in the Washington Post. "Traditional marriage is defined as the union of (1) two people of (2) opposite gender," he observes. "If, as advocates of gay marriage insist, the gender requirement is nothing but prejudice, exclusion and an arbitrary denial of one's autonomous choices," then "on what grounds do they insist upon the traditional, arbitrary and exclusionary number of two?"

Here's the answer. The number isn't two. It's one. You commit to one person, and that person commits wholly to you. Second, the number isn't arbitrary. It's based on human nature. Specifically, on jealousy.

In an excellent Weekly Standard article against gay marriage and polygamy, Stanley Kurtz of the Hudson Institute discusses several recent polygamous unions. In one case, "two wives agreed to allow their husbands to establish a public and steady sexual relationship." Unfortunately, "one of the wives remains uncomfortable with this arrangement," so "the story ends with at least the prospect of one marriage breaking up." In another case, "two bisexual-leaning men meet a woman and create a threesome that produces two children, one by each man." Same result: "the trio's eventual breakup."

Look up other articles on polygamy, even sympathetic ones, and you'll see the pattern. A Columbia News Service report on last month's national conference of polyamorists—people who love, but don't necessarily marry, multiple partners—features Robyn Trask, the managing editor of a magazine called Loving More. The conference Web site says she "has been practicing polyamory for 16 years." But according to the article, "When Trask confronted her husband about sneaking around with a long-distance girlfriend for three months, he denied it. … The couple is now separated and plans to divorce." A Houston Press article on another couple describes how "John and Brianna opened up their relationship to another woman," but "it ended badly, with the woman throwing dishes." Now they're in another threesome. "I do get jealous at times," John tells the reporter. "But not to the point where I can't flip it off."

Good luck, John. I'm sure polyamorists are right that lots of people "find joy in having close relationships … with multiple partners." The average guy would love to bang his neighbor's wife. He just doesn't want his wife banging his neighbor. Fidelity isn't natural, but jealousy is. Hence the one-spouse rule. One isn't the number of people you want to sleep with. It's the number of people you want your spouse to sleep with.

We've been this way for a long time. Look at the Ten Commandments. One: "Thou shalt have no other gods before me." Two: "Thou shalt not make unto thee any graven image … Thou shalt not bow down thyself to them, nor serve them: for I the Lord thy God am a jealous God." Three: "Thou shalt not take the name of the Lord thy God in vain." In case the message isn't clear enough, the list proceeds to "Thou shalt not commit adultery" and "shalt not covet thy neighbor's wife."

Some people say the Bible sanctions polygamy. "Abraham, David, Jacob and Solomon were all favored by God and were all polygamists," argues law professor Jonathan Turley. Favored? Look what polygamy did for them. Sarah told Abraham to sleep with her servant. When the servant got pregnant and came to despise Sarah, Sarah kicked her out. Rachel and Leah fought over Jacob, who ended up stripping his eldest son of his birthright for sleeping with Jacob's concubine. David got rid of Bathsheba's husband by ordering troops to betray him in battle. Promiscuity had the first word, but jealousy always had the last.

Thousands of years later, we've changed our ideas about slavery, patriarchy, and homosexuality. But we're still jealous. While 21 percent of married or divorced Americans admit to having cheated (and surveys suggest husbands are more likely than wives to stray emotionally and physically), only one in four women says she'd give a cheating husband or boyfriend a second chance, and only 5 to 6 percent of adults consider polygamy or extramarital affairs morally acceptable. As the above cases show, even people who try to practice polygamy struggle with feelings of betrayal.

Krauthammer finds the gay/poly divergence perplexing. "Polygamy was sanctioned, indeed common" for ages, he observes. "What is historically odd is that as gay marriage is gaining acceptance, the resistance to polygamy is much more powerful." But when you factor in jealousy, the oddity disappears. Women shared husbands because they had to. The alternative was poverty. As women gained power, they began to choose what they really wanted. And what they really wanted was the same fidelity that men expected from them.

Gays who seek to marry want the same thing. They're not looking for the right to sleep around. They already have that. It's called dating. A friend once explained to me why gay men have sex on the first date: Nobody says no. Your partner, being of the same sex, is as eager as you are to get it on. But he's also as eager as you are to get it on with somebody else. And if you really like him, you don't want that. You want him all to yourself. That's why marriage, not polygamy, is in your nature, and in our future.