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The Problem: According to the commandments of the major religions, God expects humans to have only a single lifelong romantic partner and to remain sexually faithful to them: “Thou shalt not commit adultery” (Exodus 20:14).

Yet, as any given week of tabloid headlines will tell you, humans aren’t naturally wired for monogamy. Even after we’re married or in a monogamous relationship, the sex drive continues functioning, often producing strong feelings of attraction and lust for people other than one’s chosen partner. Even celebrities and politicians in high-profile relationships, people who have by far the most to lose from being caught cheating, seem unable to resist the urgings of adulterous desire. (John Edwards and Tiger Woods are the two most famous examples in recent headlines – by the time you read this, there will probably be others.) Religion also seems ineffective at restraining lust: consider the many high-profile preachers, from Jimmy Swaggart to Ted Haggard to Jim Bakker (and many, many more), who’ve been caught in heterosexual or homosexual relationships outside their marriage.

The Solution: It’s utterly bizarre and inexplicable, on the theistic worldview, that God would create humans with overwhelmingly strong inclinations to commit an act he doesn’t want them to commit, and then punish them harshly if they fail to resist the temptations he himself implanted in them. This view makes God out to be some kind of Kafkaesque sadist who doesn’t want humans to be saved and delights in placing stumbling blocks in their path.

But it didn’t have to be this way. If God is an omnipotent architect with the power to create any kind of beings he pleases, and if God’s preferred model of sexual and romantic relations is lifelong monogamy and fidelity, it would have been easy for him to make that happen. Rather than creating human beings as we are now, God could have created a world of human beings with a different psychological makeup.

In this possible world, if entered into willingly, the ritual of marriage produces psychological and physiological change in the brain such that from that day onward, a married person experiences feelings of love and sexual attraction for only their chosen partner and no one else. The ability to feel platonic love, to form friendships and meaningful relationships based in mutual respect and admiration, would be unaffected, but the idea of falling in love or feeling lust for someone other than your partner would be as inconceivable as the idea of falling in love with a lamp or a table. In this world, adultery simply wouldn’t exist, as there would be no desire to engage in it.

The Real Explanation: Human nature was not created by God, but shaped and instilled in us by evolution. And evolution, above all else, rewards reproductive success: the drive to have as many descendants as possible, to maximize the contribution of your genes to the next generation. This is not because evolution has some sort of moral preference for this behavior, but simply because living beings that act in this way will proliferate at the expense of those that don’t, and therefore we’re more likely to be descendants of the former rather than the latter.

That being the case, it’s to be expected that many human beings become attracted to more than one person over the course of their lives. There’s no evolutionary advantage to shutting down your sex drive, while there is an evolutionary advantage to mating with anyone who might be willing. (Natural monogamy does evolve, but only on rare occasions – usually when children need the full attention and nurturing of both parents to survive.) For this very reason, if human beings were wired as I’ve described above, this would be strong evidence against evolution and in favor of God’s existence. But this isn’t what we actually find to be true.

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DAYLIGHT ATHEISM Adam Lee is an atheist author and speaker from New York City. His previously published books include "Daylight Atheism," "Meta: On God, the Big Questions, and the Just City," and most...