Most of us are convinced that we excel at being clearheaded, humane thinkers when it comes to sex. We appeal, and admirably so, to notions such as harm and consent. But since most of us aren’t anthropologists, we W.E.I.R.D. people (the anthropologist Joe Henrich’s apt acronym for “Western, Educated, Industrialized, Rich, and Democratic”) often assume a false obviousness along these lines of harm and consent that, interestingly enough, simply isn’t there. Scientists have found that since we would be harmed by a certain sex act, we presume others would be harmed as well.

In fact, cultural relativism is the most glaring sign that the lion's share of our sexual ethics is arbitrary, given that our intuitive feeling of what’s “normal” and “deviant” hinges largely on our cultural indoctrination. In the past, for instance, a proper Crow gentleman wasn’t expected to simply woo the object of his desire over a slice of homemade juneberry pie. Instead, the tradition for a man so smitten involved his crawling up to the woman’s tent in the middle of the night and fishing around with his hand under the flaps for her body. And female Crow informants explained to the anthropologists inquiring about the tradition that this manual search in the dark for her orifices was an especially romantic first move. “If he is successful,” wrote the researchers Clellan Ford and Frank Beach, “a man may by this device persuade the woman to have intercourse with him later on.” If he were successful in our society, he’d be signing his name to the sex offender registry before dawn, if he still had a hand. But in the cultural context of these Native Americans, most women, presumably, favored this custom.

Although such behavior is unspeakable to those of us living in the modern conurbations of 2013, to insist that those Crow women of yore should have felt violated by this sexual ritual, since that’s how most women feel today about this invasive sex act, is to conclude that our feelings are “accurate” and theirs “inaccurate.” As hard as it may be to step outside of our own W.E.I.R.D. heads, isn't it rather sadistic to demand others be harmed by sex acts only because those same acts would irreparably harm us?

Ethnocentrics aside, there was that tricky problem for Crow men of not knowing with certainty that the body on the other side of the flap belonged to the woman he wanted and not, say, to that of her mother—not to mention the woman’s confusion over just whose hand it was, exactly, reaching in from the outside. A similar courtship ritual in indigenous cultures from the Pacific region improved on the basic design. In those societies, a man invited a woman to have sex with him by thrusting his “love-rod”—it’s not what you’re thinking, just wait—through her window, then delicately prodding her with it. This so-called love-rod was a distinctively carved stick, and since each young man in the village would carry around his own uniquely carved love-rod during the day, he became associated with a signature relief or design. Women could therefore identify their suitor by whichever love-rod rubbed up on them, accepting or rejecting the man’s solicitous offer by pulling it in or pushing it away, respectively. It was a clever ritual. Still, it’s easy to imagine how the system might be abused. And just think how awkward it could be if, in a state of blind lust one starry-eyed night, you mistakenly picked up your friend’s curvy love-rod instead of your own. That would be a sticky situation indeed.

In many cultures, it was the woman who did the erotic bidding. And in some of those cases, the male’s consent wasn’t always so clear-cut. In northern Columbia, no matter how homely a girl may have been, she could still score the handsomest man in the village, because if she were able to literally knock him off his feet by tripping him during a ceremonial dance, he was duty-bound to have sex with her. The Lesu women of East Asia didn’t leave much room for misinterpretation, either. In those parts, a lady simply lifting up her skirt to advertise herself to a man of her choosing worked like a charm. Since she performed this brash genital display in public, a man’s refusing such a transparent offer was perceived as a slight against her.

From the vantage point of those living in other times and places, our own sexual customs can also be hard to fathom. The Tonga people of Africa, for example, could only stare in disgust on observing a married pair of Europeans kissing. “Look at them,” the ethnographers overheard the natives saying, “they eat other’s saliva and dirt.” Many struggled to wrap their heads around the prudishly cloistered appearance of the Western researchers so interested in them. In the mid-20th century, Wogeo islanders off the coast of New Guinea offered condolences to the British anthropologist Ian Hogbin. He got to the bottom of these (nearly naked) people’s concerns in a blunt conversation with a local: “He said that if he were white that he, too, would be ashamed and cover his body with as many clothes as possible.”

Acknowledging the massive influence of culture in shaping our attitudes towards sex, however, by no means implies that “anything goes” when it comes to the power of social learning. Natural selection sets limits. Even among the most sexually permissive of societies, none is so laissez faire that one’s own grandmother is viewed as fair game for a casual hookup. And although, say, certifiable “dendrophiles” (actual lovers of trees) may roam the streets as ambassadors of that rare paraphilia, you won’t find a single society in which the majority prefer to screw trees over people. In other words, if human sexuality were only a matter of social learning, then the question remains why dendrophilia—or sex with any other nonhuman entity—has never been the norm in any known society. Intercourse with a raw conifer is (I hear) somewhat painful, but pain isn’t enough to dismiss the possibility of such a culture out of hand. A bit of masochism, after all, is a common enough sexual schema in many societies. Apinajé women in Brazil reportedly bit off their male lovers’ eyebrows and spit them out during sex, while Trukese men in the Caroline Islands could expect their highly aroused wives to poke a finger sharply into their ears.

The reason that we have yet to discover a culture in which any substantive percentage of citizens are mounting trees (or horses, chainsaws, cars, shoes, etc.) is that, while there’s indeed a wide range in what is considered normal—or normal enough—across societies, for any given human population to survive, its members must first reproduce. Heterosexual intercourse is predictably, then, the most common mode of sexual expression everywhere, for that one logistical reason.

Don’t let that distract you, however, from the fact that sexual vicissitudes are indeed wildly diverse across human populations. “The amount of variation,” observed the anthropologist Leigh Minturn, “is greater than that surrounding any other biological drive. This is true not only of frequency of sexual behavior, but the kinds [of sex] … permitted or condemned.” The erotic diversity is simply breathtaking.

Some sex acts are proscribed the world over, however. From an evolutionary perspective, it should come as no surprise that all known societies censure incest, sexual abduction, and rape. But even where there are cultural agreements about these transgressions, there are huge cultural differences in how they are handled. In one society, a certain offence leads to a passing family quarrel; in another, that very same behavior results in the offender having to privately apologize to his (or her) victim and paying a fine; and in yet another society, the act in question leads to irreparable stigma, mutilation, exile, imprisonment, or even execution.

Over the years, there have been a few ambitious theoretical attempts to make sense of this bewildering array of societal attitudes toward sexual deviance, with some scholars trying to map out the intricate ways in which specific details of the environment (such as sex ratio, infant mortality, and sustainable resources) may correlate. Yet the truth is, we still know astonishingly little about why, exactly, societies differ so dramatically in these ways, and it’s mostly a game of speculation.

Much of the trouble with untangling cultural attitudes toward sex are the data themselves. The ethnographies date back to the late 19th century, an era when most scholars were corseted by Victorian-era prudery. These early scholars either ignored the subject of sex altogether, or glossed over this all-important category of human behavior with abstract models of “kinship relations” or “marriage ceremonies.” The rare details jotted down about the sex lives of those from what are now extinct cultures offered little context, and so these facts are buried without any rhyme or reason in the labyrinthine archives. Moreover, the researcher’s personal biases were frequently, and embarrassingly, evident. Referring to your subjects as “savages” was one dead giveaway of your slanted outlook.

Speaking of which, the author of The Sexual Lives of Savages, and, arguably, the most open-minded ethnographer of his time, the celebrated Polish-British anthropologist, Bronisław Malinowski, held a transparently biased attitude towards same-sex relationships. Famous for studying the Trobriand Islanders of Melanesia in the early 20th century, Malinowski was one of the few scholars writing about sex so openly. “Gradually the caress becomes more passionate,” he wrote of his subjects’ foreplay habits, “and then the mouth is predominantly active; the tongue is sucked, and tongue is rubbed against tongue; they suck each other’s lower lips, and the lips will be bitten until blood comes; the saliva is allowed to flow free from mouth to mouth. The teeth are used freely, to bite the cheek, to snap the nose and chin.” It’s as though he were scribbling notes about mongooses copulating in the weeds. Malinowski’s descriptions of the sex lives of Trobriand Islanders were so unfiltered, in fact, that British authorities accused him of trying to pervert the U.K.’s youth.

Don’t let Malinowkski’s empirical candor about heterosexual sex fool you, however. He was also an outspoken homophobe, standing out in his reviling of same-sex relationships even during a time when homosexuality was labelled a psychiatric illness. He publicly lauded the islanders’ ostracism and humiliation of gays and lesbians, encouraging his Western readers to take notes from the Trobrianders on how to deal with such undesirables in their own societies. “How far it is true,” he reflected admiringly on this foreign culture, “that homosexuality is more efficiently eradicated by derision than by heavy penalties.” Malinowski’s antigay legacy hasn’t sat well at all with the historian Peter Stearns. “Here was an ethnographer,” wrote Stearns decades after Malinowski penned those cruel lines, “who was concerned that homosexuality was growing in respectability in his own society, who wished that it could be entirely exterminated from human society, and who was delighted to imagine that he had found a people who had managed to do so by making fun of it and by providing ‘wide and varied opportunities for normal intercourse.’”

What Malinowski fell prey to in his line of reasoning about homosexuality, and what many otherwise intelligent people still succumb to today, is the naturalistic fallacy—the philosophical error in which “natural” is mistakenly conflated with “good.” There are many things that are natural that are immensely harmful, and vice versa, many unnatural things that have made our lives far more pleasant and positive. Naturalness connotes no intrinsic moral value at all, and normal is only a number.

The many differences in sexuality found across human societies are impressive, as I think you’ll agree. Yet where does this leave us in our ability to discern an “objective morality” out there in the universe—in this case, sexual rights and wrongs that exist independent of our own enculturated biases? If you take God out of the picture (and there’s certainly no obvious reason to include Him, evolutionarily), does an objective morality even exist?

Quite simply, no. Through the rhetoric of righteousness, we're bullied into subscribing to the delusion that it does—but it doesn't. We'd also do well to abandon our strange preoccupation with the meaningless question of what is “natural” in human sexuality. Unless we wish to invoke a Creator God who preconceived our loins and prescribed our genitals for reproduction and nothing more, “natural” is a useless construct when it comes to sexual ethics. To gain any moral traction on such slippery issues, while also keeping a clear view of the sheer range of erotic diversity displayed over time and space, we’d do better to devote our efforts and intellects to defining harm in a way that applies not to us as onlookers, but to the subjective minds of those involved.

I discuss this issue in more detail, and much, much more, in my new book Perv: The Sexual Deviant in All of Us, which will release on October 8, 2013. Follow me @jessebering (#DailyDeviant). For more on all things deviant, and to find out if I'll be visiting a city near you for the Perv book tour, visit www.jessebering.com.