The Great Cunnilingus Debate

Do Men Use Oral Sex to Tell if Their Wives or Girlfriends Are Cheating?

This article was originally published in Le Point on 07.07.2025

It doesn’t escape me that a gay man writing about cunnilingus is peak irony. It always seems on the tip of my tongue but I struggle just to pronounce the word properly. At least nobody can accuse me of being too close to the topic to view it clearly. And when it comes to understanding why men the world over, from the beginning of time, have invested so much of their erotic energies into pleasuring women this way, perhaps a bit of distance is helpful, as succumbing to an air of pure, sniggering puerility is arguably our greatest obstacle.

Interrogating the whys of human behavior, especially sexual behavior, is, of course, the academic remit of evolutionary psychologists, and although a thing may seem as straightforward as a tongue in a hole, reality is always fuzzier than expected. From a purely reproductive perspective in which sexuality is driven by unconscious behavioral algorithms designed by natural selection to maximize genetic transmission, that’s just not the right organ to be inserting. Cunnilingus alone is no more effective than, oh I don’t know, anal sex.

Yet when we view the behavior—and the psychological mechanisms behind it—from a wider angle of what biologist Randy Thornhill describes as “the co-evolutionary race between females to control conception and men to circumvent that control,” cunnilingus comes intriguingly into focus as a potential adaptive strategy. The central idea is this: due to the very real threat in the ancestral past of a man being cuckolded (raising and investing in another male’s offspring, thinking it’s his own), human males evolved a string of infidelity-detection tactics. A man performing oral sex on his female partner—especially during estrus, when the likelihood of conception is highest—reasoned Thornhill, may actually be a form of stealth information-gathering about her recent sexual activities. Like a dog sniffing a fire hydrant to sleuth out any other visitors, men’s interest in and time spent on cunnilingus should be proportionate to his perceived risk of being cuckolded. The evolved psychology behind this, needless to say, would predate the invention of soap (and that horrible douche), so what we see now with our sexual affinities would be an artifact of that more malodorant past.

“There is evidence that men with partners with multiple, simultaneous boyfriends show a greater peak in mate guarding during estrus than men with partners who report dating only one man,” Thornhill writes. “Perhaps cunnilingus would reveal the same pattern. Cunnilingus during estrus . . . might function to detect by taste/olfaction another man’s ejaculate.”

Now that I’ve got your attention—seriously, reread those last lines to fully weigh the implications of his claim, that going down on your missus is mostly just a tactical opportunity for you to taste and smell if another man’s semen has been freshly deposited in her vagina—let’s look at the evidence for (and as it turns out, against) Thornhill’s oral sex hypothesis.

Some of the clues are indirect. We’re not the only species to indulge in the act. Bonobos, orangutans, and lemurs do it too, and it’s even been observed in fruit bats. (“Fellatio in Fruit Bats Prolongs Copulation Time” may be one of my favorite titles ever for an academic article.) Male lemurs spend more time licking and sniffing down there during female estrus. And human males judge vaginal fluids from women at peak fertility as more pleasant-smelling.

Then there’s that copious slew of data from sperm competition theory, which suggests that men’s mating psychology was shaped to be especially vigilant against the genetically costly risk of cuckoldry. This, say evolutionary psychologists, might take the form of male jealousy and various “mate-guarding” behaviors, with men attempting to minimize their partner’s opportunities to copulate with other men, or even physiologically calibrating their level of sexual desire based on the estimation of her cheating probability. We know, for instance, that when a couple is separated for a few days, absence not only makes the heart grow fonder, but the phallus grow harder: men report deeper and more vigorous thrusting than normal at the first copulation after reuniting, and they also ejaculate more sperm. That makes sense, say scholars, given that sperm can remain viable in the female reproductive tract for up to five days after intercourse, and, well, she has been off the radar for a while. “Inseminating more sperm increases a man’s chance in the ‘sperm raffle’ to fertilize his partner’s ova,” explain psychologists Michael Pham and Todd Shackelford from Oakland University. When you pair that logic with the distinctive shape of the human penis—a plunger-like “semen-displacement device” with a coronal ridge designed to extract a recent competitor’s sperm from a vagina, replacing it with one’s own—you can see how the, um, pieces fit together.

With that theoretical framing in mind, reasoned Pham and Shackelford in a study published in Personality and Individual Differences back in 2013, perhaps cunnilingus is indeed just as Thornhill suggested, a way for men to detect their partner’s infidelity. “Cunnilingus may allow men to taste and smell rival semen near or within the vagina, providing cues to a woman’s sexual history,” write the authors. “[If so], then men at greater recurrent risk of sperm competition will report greater interest in performing cunnilingus on their partner, and will perform cunnilingus for a longer duration, to better detect rival semen.” I hasten to add that men need not be consciously aware of any such implicit motive behind their lustful deeds; they can be as obtuse as a horny lemur and not even know what the words cuckoldry and cunnilingus mean for the sex act to still function in an adaptive way.

Yet even factoring that in, the precise mechanisms aren’t immediately clear to me. Let’s say you do happen to taste another man’s ejaculate while giving your partner’s genitalia a thorough tongue-lashing. Then what? But we’ll get to those iffy explanations in a moment.

To test their predictions, Pham and Shackelford conducted a survey with 231 men “in a committed sexual relationship lasting at least one year” and asked them about their cunnilingus proclivities. Long story short, their hypotheses were supported. Even after controlling for relationship length, relationship satisfaction, and duration of sexual intercourse, it was recurrent risk of sperm competition that best predicted men’s interest in and how much time they spent performing oral sex on their partners. That key variable of sperm competition risk was defined in the study as how physically and sexually attractive they thought their girlfriend or wife was, in their own eyes and in the eyes of other men. (It's not that more attractive women are more prone to cheating, but other men will naturally be more solicitous toward them, affording greater opportunities for surreptitious mating.)

But the devil, as they say, is in the details, or in this case the semen. It’s not obvious how sniffing another man’s ejaculate in your partner’s crotch—or thinking you’ve done so, which begs the question of whether men can accurately detect through taste or smell a stranger’s semen among a melange of other bodily emissions—would translate to increased reproductive success. I’m not aware of any studies demonstrating this superpower, and I can imagine it might be challenging finding agreeable male participants. But let’s assume such an above-chance ability to discern sexual interlopers does exist. Again, what then? For many couples, I suspect it would lead to an abrupt halt in bedroom festivities and a really awkward conversation. I suppose it could also make men more lascivious than ever, in the same way that men tend to get more turned on by porn featuring other males with women than by scenes of just lesbian sex. That is, the “presence” of another male would heighten arousal and, therefore, deeper, more vigorous thrusting in the sex that follows, leading to foreign semen displacement and maybe even a winning entry in that aforementioned sperm raffle. That said, and maybe it’s a straight male thing, but I still have trouble imagining that another man’s seed landing rudely, unexpectedly on your taste buds would be anything but a turnoff.

Pham and Shackelford do suggest two alternative explanations for their findings, however. Even if you remove the other male’s seminal load from this hypothetical vaginal scenario entirely, they note, cunnilingus could function as a pre-emptive adaptation. The authors point out that women who first receive oral sex are more likely to experience an orgasm in the “copulatory event” that soon follows, and since female orgasms help to retain more sperm (and thereby to increase the chances of fertilization), this sort of oral pump-priming of female arousal is adaptive in its own right. Also, in the event that another man does happen to have sex with her shortly thereafter, your own sperm cells, safely “retained” in her cavernous vaults, would be harder for him to displace with his penis. “Men who induce their partner’s orgasm through cunnilingus and then soon after ejaculate into their partner’s reproductive tract,” explain the authors, “will have more sperm retained in her tract, thereby providing them with a competitive advantage in any ensuing sperm competition.”

For those who prefer a slightly more vanilla explanation for cunnilingus, the authors also offer the “sexual satiety hypothesis,” which boils down to men simply doing everything they can to satisfy their partners to keep her from straying, since sexually dissatisfied women are more likely to be unfaithful. Previous research, in fact, found a positive correlation between frequency of cunnilingus within relationships and women’s sexual satisfaction. I mean, I wouldn’t know personally, but I imagine it feels pretty damn good.

You’ve probably been asking yourself about the other oral sex. Are women motivated to perform fellatio as a way to detect if their boyfriends and husbands have been dipping their you-know-whats in who-knows-who? In a follow-up study, published the same year in the same journal, the authors repeated their study with a female sample. But in this case, there was no link at all with infidelity risk (measured as their male partner’s perceived attractiveness to other women, as well as to themselves) and interest in or time spent giving him a blowjob. “Men but not women at greater risk of partner infidelity report greater interest in, and spent more time performing, oral sex on their partner,” the authors explain. The best predictor of a women’s interest in fellatio was the duration of the sexual encounter: basically, it was just something in her repertoire, bound to happen at some point during the long steamy session.

So why is female oral sex, but not male oral sex, explainable as an evolutionary adaptation? The authors have a few speculative ideas. First off, there’s an asymmetrical disease risk between the two oral activities. “STDs are more easily contracted from fellatio than from cunnilingus,” they note. “This greater cost of oral sex may have placed a ceiling on women’s but not men’s interest in, and time spent, performing oral sex.” There’s also, they point out, the matter of viscous matter. “The differences in fluid quantity left behind by a rival (i.e., semen from a rival male vs. vaginal fluids from a rival female), may influence the effectiveness of, and motivation to use, oral sex as a means to detect infidelity.”

If you’re having trouble buying any of this, you’re not the only one. In 2022, an independent research team led by the psychologist Rebecca Koessler at Western University, Canada attempted to replicate Pham and Shackelford’s original cunnilingus findings from 2013, and failed to do so. Koessler et al.’s study, also published in Personality and Individual Differences, included a larger sample size of both men and women. Importantly, they added an additional—and what they believed was an improved—measure of actual infidelity risk. Rather than just asking how attractive the participants found their partners (and how attractive they thought potential rivals found their partners, which implied a relative risk of cheating), they also asked participants if they suspected their partner of infidelity.

In this new study, the latter measure had no bearing whatsoever on oral sex habits within the relationship, for either men or women. This, Koessler and her coauthors reason, throws a wrench in the works for Pham and Shackelford’s infidelity-detection hypothesis of cunnilingus, and they aren’t shy about saying so. “We advise researchers to . . . proceed with caution when exploring the proposed evolutionary links between partner infidelity risk and oral sex behaviors.” But their study design wasn’t exactly flawless either, it seems. For example, when questioning participants about their most recent episode of oral sex, the wording was ambiguous—“how long did you have oral sex with your partner?” Whether participants understood this to mean giving or receiving clouds the picture here quite a bit.

Personally, I remain unconvinced that cunnilingus is an adaptation, or is not an adaptation. It’s still an open question, namely because none of these correlational studies addressed the most obvious question of all, which is whether human beings are in fact the preternatural scent hounds of seminal fluids they’d have to be in order for this infidelity-detection mechanism to work. That sort of blind taste (or smell) test will turn most people’s stomachs, and is likely a big part the reason it’s never been done. But hey, it’s a dirty business, science.

Like what you read? Toss a tip in the jar—any amount that makes sense for you—so I can keep overthinking things on your behalf.