It was over dinner when it dawned on Matt that he might be having a threesome that night. He and his partner were out at a restaurant with her friend, another bisexual woman. The friend had made her attraction to Matt’s partner clear; Matt’s partner, for her part, had regularly joked about “how funny it would be if …”.

The idea of a threesome came up again over dinner. “Without skipping a beat, or even consulting me, my partner just kind of went for it: ‘Have you ever thought of having sex with a mixed-sex couple?’”

Matt quickly picked up his cellphone – and started playing Candy Crush.

It wasn’t that he was against it, he says. “I just knew there was nothing I could say or do to help my partner negotiate – so there I sat, trying to play it cool and seem like I wasn’t paying too much attention.” Thirty minutes later, they settled the bill – and Matt’s partner quietly informed him that they would be having a threesome that night.

A threesome is the most common sexual fantasy among Americans, according to a survey of 4,175 individuals carried out by the Kinsey Institute sex researcher Dr Justin Lehmiller for his book Tell Me What You Want. Of the highly diverse sample aged 18 to 87, 95% of men and 87% of women said they had fantasized about sex with multiple partners. But among those who had acted on it, Lehmiller’s research revealed threesomes were also “the fantasy that was least likely to turn out well” – not least because all three individuals involved might have had very different expectations.

“Most people don’t have a script for how a threesome should go,” says Lehmiller. “It’s very easy to think, ‘This sounds like a great idea’ … but the reality is often very different from the way we picture it in our heads.”

In 2017, a US study with a nationally representative sample of just over 2,000 people found that 18% of men and 10% of women recorded having had a threesome in their lifetimes. Studies in the US and Canada have established that about one person in every five has engaged in it in some form, at some point in their lives – making it about as common as owning a cat.

Those who have not had a threesome may imagine it as a straightforward matter of multiplication: the connection and sensation of sex between two people, multiplied by 150%. Couples especially may regard the third party as auxiliary, an add-on to augment their pleasure. But this glosses over the vast potential for complexity, beyond FFM or MMF gender composition. (The fact that FFF or MMM are barely spoken of speaks for itself.)

Participants may also differ on how much prior discussion is desirable, what is and isn’t out of bounds, and for whom – even on who is to be the center of attention. “Different people can be into threesomes for very different reasons,” says Lehmiller. In fact, the most common sexual fantasy may also be the most misunderstood.

Last year Dr Ryan Scoats, of Coventry University, published Understanding Threesomes – the first in-depth study on threesomes in 30 years. He had found that the motivations went far beyond straightforward novelty-seeking, from ticking it off a sexual “bucket list”, to settling psychological debts – for example, by levelling the score with an unfaithful partner. One woman who had cheated on her husband in a threesome later repeated the experience with him as payback.

“It can be an absolute sex romp,” says Scoats. “It can be about power, it can be about fun and excitement, it can be about alcohol. It can be that it was just there, and ‘Why not?’ And it can be all of these things, and none of these things.”

Even with the caveat that people are generally less likely to volunteer to talk to researchers about their negative experiences, Scoats’ biggest surprise was the “shocking” diversity of those he recorded. “And maybe also, the – what’s the right word? The mundanity. For many of these people, it just wasn’t really a big thing.”

Some study participants told Scoats that they had gone ahead with it primarily for someone else’s benefit – a motivation he termed “sexual altruism”. It might be that their partner had a threesome fantasy that they didn’t necessarily share, or that they wanted to act on same-sex attraction within a mixed-sex relationship (especially common among women).

On the darker side, it can also be a bargaining chip. “Odds are, your partner has a threesome fantasy,” says Lehmiller. “It could be a way of getting some leverage in the relationship.”

The thought may land awkwardly at a time when the importance of clear and enthusiastic consent is increasingly – and rightly – being foregrounded in discussions of sex; when it is suggested that the only sex anyone should be having is sex they enthusiastically seek.

“There’s been a lot of coercion when it comes to group sex and women’s participation,” says Katherine Frank, a cultural anthropologist and author of Plays Well in Groups: A Journey Through the World of Group Sex. “If you think about a threesome between two gang members and one of the girls who hangs out with the gang – that threesome could be just as much about affirming a hierarchy among the men as it has anything to do with her.”

Some of Scoats’ case studies paint a similarly grim picture, with women reporting feeling like they “couldn’t really say no” or that it would be “more awkward to back out” than to go ahead. (Some male participants also expressed regrets, particularly after having involved a friend.)

But more often with the cases he identified as altruistic, the less enthusiastic party had been “not that fussed”, rather than actively torn; they went through with the threesome because they knew it would mean a lot to their partner.

Scoats likens it to going to a partner’s family home for the holidays. “If we look at other areas of our lives, we can probably find a good number of examples of doing things that you might not be that up for. It just feels strange when we have these conversations in terms of sex.” (Even therapists are split on the merits of “maintenance sex” for couples as a means of keeping their physical relationship alive.)

Frank says it is common for one half of a couple to “take one for the team” more than the other. “But doing that for your partner is not the same as coercion. In some cases it could be, but it’s just that there’s an imbalance in desire.” Plus, that imbalance often shifts over time – for example, a woman’s initial reluctance to go to erotic parties may change once she discovers how in-demand she is there, men being “a dime a dozen”. (For that reason, MMF threesomes happen more often than FFM, Frank says.)

Scoat suggests the test should be of impact. “If it is something you actively do not want to do, that you feel is going to be harmful to you in some way, then that’s probably a good reason not to do it. But if you think it’s just going to bounce off you, it’s not really going to affect you positively or negatively, but someone else will enjoy it – is that problematic?”

The truth is, many threesomes are driven by a desire for novelty. For younger people taking what Scoats calls the “consumer sexuality” view, that may be to maximise their experience before settling down into a relationship. But Lehmiller’s data shows college students are actually the least likely group to fantasize about threesomes.

Instead, Lehmiller found, interest in threesomes peaked around age 40 and remained high for about the next 20 years before declining. “Most of these people are in long-term, monogamous relationships, and they are looking at threesomes as a way of injecting novelty into their sex life,” he says.

“Opening up” a monogamous relationship is often perceived as a threat to its long-term stability, either by reflecting a weakened commitment or by giving rise to jealousy. That can certainly be the case, says Frank. “You can’t control what happens with two people. What makes you think you could with three?

“People say, ‘You need to overcome your jealousy’, and that can sound really great when you’re sober in the daylight, but a lot of sex is subconscious. Something can suddenly hit you: ‘He moved her hair in a certain way’ – those things can trigger a jealous response that’s not even rational, that may be fleeting or intense.”

In general, Frank cautions against having too many expectations of how threesome might go, especially for the first time: “The fantasy almost never lives up to the reality.”

Matt says his partner, on arriving back at his house, demonstrated some “pretty amazing savvy” by excusing herself to the bathroom, specifically to give him and their friend some time alone together. “She recognised in the moment that if me and the other woman didn’t build some physical chemistry, it would become increasingly difficult to incorporate me later on.”

But he was surprised by how jealousy did and didn’t manifest in the bedroom. Beforehand, Matt says, he would “have bet major money” that some acts would have provoked his partner – and he would have lost. Those that he thought relatively minor had more of an impact, necessitating “a bit of in-the-moment care to make her feel valued again”.

And afterwards, the friend had seemed a bit “bummed out” when there wasn’t room for her to sleep over. “My bed was only a double,” says Matt.

Logistics aside, much of the opposition to threesomes is rooted in society’s centering of monogamy, which continues to be rewarded with legal and cultural benefits even as marriage rates have fallen. Though consensual non-monogamy is increasingly visible in media and research, many people are put off exploring it themselves because of the wide-reaching social stigma.

Dr Amy Moors, an assistant professor of psychology at Chapman University and Kinsey research fellow, says her research has shown that people practicing consensual non-monogamy are viewed negatively in just about every way – from their safe sex practices to their effectiveness as parents.

Subsequent research found that people found polyamory (often simplified as loving more than one person) marginally more palatable than arrangements allowing for sex without emotion. “What we’re finding is robust stigma, but not stigma like ‘women don’t belong in the workplace’ or ‘smoking’s gross’ – it is like, ‘This is morally bankrupt behaviour’,” says Moors. “No-strings-attached sex – that’s where our sex-negative culture gets the best of us. Like: ‘How could you do that?’”

Yet at the same time, against this backdrop of moral panic, there is growing awareness that monogamy is harder than most of us have been led to believe, and maybe even entirely unrealistic. “We have to acknowledge that roughly a quarter of people end up sexually unfaithful in a relationship,” says Dr Heath Schechinger, a therapist and co-chair (with Moors) of the American Psychology Association’s consensual non-monogamy taskforce.

“The No 1 reason, year after year, that people pursue couples counselling or divorce is because of infidelity – so we can either continue to try to put a square peg in a round hole, or we can start asking open-minded questions.”

Schechinger points to the psychotherapist Esther Perel’s framing of our co-existing and contradictory desires for security and novelty: “I see more and more couples that seem to be wrestling with this idea.” (Perel has also said women get bored with monogamy faster than men do.) A threesome – being seemingly more straightforward than a fully open or ongoing arrangement – might present as a means for them to have it both ways. Schechinger says that, for some, the appeal of a threesome might be as “something that you can do with your partner”: “You still have that access to other people while also doing it within the comfort of your relationship.”

There are too many traditions and laws surrounding monogamy for it to ever be superseded, Schechinger says – but he expects a “monogamish” model, as championed by the sex advice columnist Dan Savage, to become more normalised. “There will be more space for people to talk about how to get their novelty needs met within the context of their relationship.”

After all, Schechinger says, it is a high degree of consent, comfort and open communication that seems to drive relationship satisfaction, more than whether it is monogamous or not.

In Matt’s case, his threesome was now eight years ago. He is no longer with his partner, and he has not repeated the experience since. But there was a particular moment, he remembers fondly, when he felt “totally liberated” from pressures to perform and the script that sex typically follows, in a way that he had never felt before.

“I still think about this feeling often,” he says. “To this day.”