I thought he was kidding. This couple are my "model marrieds", true equals who share the housework and child care, communicate openly and prioritise each other's careers. The best friends of happy-couple cliché. Earlier in the evening, I watched them work together in the kitchen, cheerfully cooking and cleaning. When their six-year-old woke up with a nightmare, they wordlessly agreed that he would be the one to soothe her. "Exactly," my boyfriend said. "Least likely." Marriage is hardly known for being an aphrodisiac, of course, but my boyfriend was referring to a particularly modern state of marital affairs. There's more gender-fluidity when it comes to who brings in the money, who does the laundry and dishes, who drives the car and braids the kids' hair, even who owns the home. They aspire to what's known in the social sciences as an egalitarian marriage, meaning that both spouses work and take care of the house and that the relationship is built on equal power, shared interests and friendship. But the very qualities that lead to greater emotional satisfaction in peer marriages, as one sociologist calls them, may be having an unexpectedly negative impact on these couples' sex lives. As a psychotherapist who works with couples, I've noticed that no matter how much sink-scrubbing and grocery-shopping the husband does, no matter how well husband and wife communicate with each other, no matter how sensitive they are to each other's emotions and work schedules, the wife does not find her husband more sexually exciting, even if she feels both closer to and happier with him.

One couple I saw had been married for five years and wanted to work out some common kinks related to balancing their respective jobs, incomes and household responsibilities in, as the wife put it, "an equal way". Over the course of treatment, the couple reported more connection, less friction and increased happiness. One day, though, the husband brought up a new concern: his wife now seemed less interested in having sex with him. He wondered, why did she appear less interested now that their relationship seemed stronger in all the ways she wanted? "I'm very attracted to you," she reassured him earnestly. "You know when I really crave you? It's when you're just back from the gym and you're all sweaty and you take off your clothes to get in the shower and I see your muscles." Her husband countered by saying that this very situation had occurred that morning, but that his wife became irritated when he tossed his clothes on the floor, which led to a conversation about his not vacuuming the day before, when she worked late. He had worked late, too, which accounted for the lack of vacuuming, but still - she hated waking up to a messy room, and it was his turn to vacuum. "Right," she agreed. "I wasn't focused on sex, because I wanted you to get out the vacuum." "So if I got out the vacuum, then you'd be turned on?"

His wife thought about it for a minute. "Actually, probably not," she said slowly, as if hearing the contradiction even as she was speaking it. "The vacuuming would have killed the weight-lifting vibe." When I ask Esther Perel - a couples therapist whose book Mating in Captivity addresses the issue of desire in marriage - about the role sexual scripts play in egalitarian partnerships, she explains it like this: "Egalitarian marriage takes the values of a good social system - consensus-building and consent - and assumes you can bring these rules into the bedroom. But the values that make for good social relationships are not necessarily the same ones that drive lust." In fact, she continues, "most of us get turned on at night by the very things that we'll demonstrate against during the day." The couples I see in therapy are eager to talk about levelling the domestic playing field, but tend to feel awkward about bringing the concept of power into conversations about sex, mostly because it can feel so confusing. One woman in her late 30s, for instance, who has been in a peer marriage for 10 years, said during couples therapy that when she asked her husband to be more forceful and "rougher" in bed, the result was comical. "He was trying to do what I wanted," she explained, "but he was so ... careful. I don't want him to ask, 'Are you okay?' I want him not to care if I'm okay, to just, you know, not be the good husband and take charge." And yet, she said, his caring and his concern that she's okay with what he's doing are what she loves so much about him in every other area of their marriage, ranging from which brand of toilet paper to buy to what to feed their children to where their money is spent and which nights each of them can stay late at work. "I don't want him to take charge like that with anything else!" she said.

As author and journalist Daniel Bergner has written in his book What Do Women Want?, many studies show that women often report fantasies, like those involving submission, that tend to be inconsistent with our notion of progressive relationships. But sexologist Pepper Schwartz says that while women may have always had these types of fantasies, now they have permission to give voice to them because of how much power they have in real life. "The more powerful you are in your marriage, and the more responsibility you have in other areas of your life, the more submission becomes sexy," Schwartz says. "It's like: 'Let me lose all that responsibility for an hour. I've got plenty of it.' It's what you can afford once you don't live a life of submission." Men, of course, can feel just as uneasy with overt expressions of power in marriages that are otherwise based on equality. During a couples session, one woman in her early 40s said that it wasn't until she came across some porn scenes her husband had viewed online that she felt comfortable telling him about her fantasies, which happened to be very similar to what she found. She thought he'd be thrilled, but although he enacted the scenes with her, she was surprised by his lack of enthusiasm. "I felt like he was just doing it because I asked him to, not because he wanted to," she said. It bothered her that her husband acknowledged being turned on by watching the fantasy online but not by doing it in real life with her. "I felt so rejected," she said. "I told him, 'I want you to want to dominate me,' but he said he just doesn't see me that way, that he doesn't see us that way." For this couple, the experiment felt so awkward that they quickly reverted to their routine: sex in the usual roles and positions during a window between 10.30pm and 11pm, when they were both tired but not yet asleep. When I turned to her husband for his perspective, he seemed relieved that he could express his puzzlement.

"It's nice," he said about the sex they have. "It's not superhot all the time, but it's really nice. I'm attracted to her. I don't know what she expects. If I don't clean up the bathroom, if I don't give her equal time with her work, if I make a decision without consulting her, she wouldn't want that. I'm so used to interacting with her as an equal - and I also want that - but I like what we have, and occasionally I like getting the other stuff on the internet. "Isn't being a good husband and father and wanting to have semi-respectful sex with my wife enough? Before we got married, we always said we'd have a 50-50 marriage, and you'd think that would be great for our sex life, but instead it's the one area where we're having trouble." He took a deep breath before adding: "I know what a 50-50 marriage should be like. But what is 50-50 sex supposed to be like?" Sex in any marriage is idiosyncratic and complex - and if it's consensual and enjoyable, it's nobody's business, frankly. But the idea that married sex should be steamy is reflected in our culture. Take the fascination with MILFs - consistently one of the most-searched porn categories and a staple in mainstream media - in which mothers are depicted as alluring and sexually lively. In the past, a fantasy woman may have been the young, single secretary; now she's the middle-aged mother of three with a graduate degree. In a way, this might seem like an encouraging shift for married mums. Instead of becoming invisible, we're wanted and capable of doing it all: work, play dates and having hot sex lives. But these sorts of portrayals also create a false sense of reality. "The passionate marriage used to be a contradiction in terms," Esther Perel tells me. The quality of sex in marriage - and not just the frequency - is a relatively new conversation that has come about with more egalitarian marriages. In today's marriages, she says, "we don't just want sex; it has to be intimate sex. It has to be transcendent and self-actualising."

Which brings me back to the dinner party where that husband made a joke about net porn. The conversation started innocuously enough, with the husband making the observation that with men and women both balancing the responsibilities of work and home, even sex needs to be outsourced sometimes. By day's end, he said, men feel so worn out that they, too, "get headaches" because they don't necessarily have the energy to make sex happen or, more specifically, to make it happen in the way their wives want it to. The modern marital tableau, he quipped, is two overwhelmed people trying to relax before bed: he on Pornhub, she on Pinterest. Then they kiss and go to sleep. Porn, of course, doesn't tend to be about reciprocity. "Here's the essence of porn," Terry Real, a couples expert, tells me. "What will you never see in a porn video? 'Honey, I don't like that.' 'Could you stop doing that?' 'Could you take a shower first?' The archetype of the porn queen is that she's a woman who derives sexual pleasure by giving the man pleasure, and - here's the key - everything he does is absolutely perfect! What you don't see in porn is anything that needs to be negotiated, the woman having needs of her own or the roles being reversed." Women are now coming into marriage with sexual histories and experiences on par with men's, leading to expectations that are difficult to replicate in any marriage, especially now that people live longer and will be having sex, presumably with the same person, for decades more. Similarly, older couples who can now wait and marry for love have less time together during their sexual primes and, if kids are in the plan, they may even miss that year or two of newlywed abandon. (Ask a 40-year-old couple trying and failing to conceive how much fun the sex is.) Fifty-year-olds of the past were often grandparents without great expectations about their sex lives. Now those same 50-year-olds might have a 10-year-old, placing them in a life stage formerly occupied by people in their 30s and subjecting them to pressure to maintain the culture's view of "youthful sexuality" in marriage, especially with the ubiquity of Viagra and Estrace. Helen Fisher, a senior research fellow at the Kinsey Institute, notes that even people who are satisfied with their sex lives often crave more nowadays. She tells me about a study she conducted that asked participants who had had affairs why they did so. Fifty-six per cent of her male subjects and 34 per cent of her female subjects said they were "happy" or "very happy" in their partnerships but cheated anyway.

While past research has shown that men have higher rates of infidelity than women, those rates are becoming increasingly similar, particularly in younger people in developed countries, where recent studies have found no gender differences in extramarital sex among men and women under 40. This may be because younger women are more likely to be in peer marriages - and conditions in peer marriages make female infidelity more probable than in traditional ones. A large US study in the late 1990s found that women who were more educated than their husbands were more likely to engage in sexual infidelity than if they were less educated than their husbands. Studies also find that people who work outside the home and whose partners remain in the home cheat more - and the traditional gender roles in this situation are now frequently reversed. As women increasingly work in professions that are not female-dominated, they have more sexual opportunities with peers than ever. Ian Kerner, a sexuality counsellor and the author of She Comes First, works a lot with stay-at-home dads and men who work from home. "One thing I hear a lot is that in theory they're really happy balancing flexible work with stay-at-home responsibilities, while their wives are out working full-time in corporate jobs," he says. "But at the same time, a common complaint is that Mum comes home and feels guilty for being away all day, and so much time has to be made up connecting with the children, who take first priority, that these dads feel lost in the mix." In many couples, Kerner says, the wives start to feel disgruntled because their husbands get to see more of the kids, and the husbands, whose wives are controlling more of the spending, start to feel "financially emasculated". Sometimes, he says, a vicious cycle begins: the husband feels marginalised and less self-confident, which causes the wife to lose respect for and desexualise him. Under these circumstances, neither is particularly interested in sex with the other. On an emotional level, "kindred spirits" sounds lovely. But when it comes to sexual desire, biology seems to prefer difference. Now that women do and have and are many of the things that they used to seek in their partners, Pepper Schwartz says that a result can be something more sibling-like than erotic. Her research likewise suggests that too much similarity in egalitarian marriages leads to boredom and decreased sexual frequency. "When you're best friends with your partner, there's less frisson," Schwartz says. "Introducing more distance or difference, rather than connection and similarity, helps to resurrect passion in long-term, stable relationships."

Yet a married friend who describes his wife as his "best friend" says he is happy to take a high degree of simpatico over a high degree of sexual pull. "I can walk down the street and be attracted to 10 people and want to have sex with them," he says, "but it doesn't mean they're going to make me happy. It doesn't mean I'd want to live the day-to-day with them. There are always going to be trade-offs." Edited version of an article first published in The New York Magazine. Lead-in photograph by Craig Cutler. Like Good Weekend on Facebook to get regular updates on upcoming stories and events.