The first time I laid eyes on Debbie, I was gobsmacked: golden hair, full lips, perfect body beneath a lemon-yellow summer dress.

Until that moment, I’d thought love at first sight was Disney propaganda. But the idea struck without warning: Marrying this woman would make my life perfect.

Seduction didn’t work, so I took pleasure in verbal jousts, a couple of grad students teasing and laughing. Months of this. Her sense of humour was as wondrous as her looks. We slowly morphed into friends without benefits.

Nearly a year after our first encounter, Debbie mailed me a photo of her topless by a swimming pool. She’d started having dreams about me, her note said.

We were married within the year. Now my life was indeed perfect, the sex a nonstop wet dream from which I hoped never to wake up.

Eventually, of course, I did wake up.

We’ve all heard that old chestnut: If you put a bean into a jar every time you have sex your first year of marriage, then take a bean out every time thereafter, the jar will never be empty.

Scientific validation for this is sketchy, but people who study long-married folks—and long-married folks themselves—agree: Chances are, you’ll be getting less as the years wear on.

But how much less?

“It’s the number one question I’m asked,” says one such researcher, Brian Willoughby, Ph.D. “How often are you supposed to have sex in a healthy marriage? I immediately say you’re thinking about sex wrong—there’s no magic number for any given couple.”

Fine, but what if this not-so-magic number is slouching perilously close to zero? This can’t be healthy, right?

Wrong—it can, say the researchers and the therapists who listen to stories of low-sex marriages. It just happens, though few couples will admit it except in therapy sessions, says sex therapist Stephanie Buehler, Psy.D.

Few are confessing to researchers either. Nonetheless, two major studies have made educated guesses: Somewhere around one in six American marriages are “sexless,” depending on how that term is defined.

One study found that 16 percent of married couples hadn’t had sex in the month prior to being contacted for the National Survey of Families and Households.

And another study by University of Chicago sociologist Edward Laumann, a leading researcher in his field, concluded that about 14 percent of married men and 15 percent of married women had had little or no sex within the previous year.

One study found that 14 percent of married men hadn’t had sex in the past year.

If you’re squirming with recognition, maybe you should relax.

It’s possible that sex droughts are hitting younger couples these days, but generally, if you’ve settled into a comfortable pattern, your therapist would nod understandingly. It’s called married life.

Infrequent sex may be a vestige of our evolutionary past, when a sexual shutdown naturally occurred in what is now early middle age.

Then there’s passion-sapping 21st-century life: work, distractions, children, stress, medication, porn, and that murky cocktail of exhaustion and indifference.

All of this can conspire to make both of you roll over and fall asleep without even thinking about sex—except, foolishly, to compare yourself to everyone else, who you’re sure is having more than you.

It’s hard to pinpoint when our sex life began to wane.

When our first son turned 3, Debbie and I were 35 and 39, respectively—what our ancestors would have considered a ripe old age. After our second son’s arrival—a trying conception involving fertility treatments—our sex lives took a predictable sabbatical.

Between sleep deprivation, the demands of a newborn and his rambunctious brother, and the financial challenges of a growing family, the last thing either of us craved was more exercise, even of the pleasantly horizontal variety.

My wife and I are not alone. Among Americans, Laumann says, “only 5 percent are having sex four or more times a week.” Fully one-third of people 18 to 59 have sex less than once a month (or not at all because they lack a partner).

For another third, it’s about once a week, and for the rest maybe twice a week or a bit more.

Moreover, Laumann thinks rates of sexual inactivity have remained steady over recent decades.

Some problems have persisted since the invention of marriage: health disorders that make sex impossible, loss of desire, and relationship conflicts that douse smoldering embers.

All expected. What may be new is that these troubles are starting sooner, among couples in their 20s and 30s, some therapists report.

Their frequency is dropping to a rate where they feel like they’re sexually inactive. Desire has dropped too.

“I’m seeing it in young, attractive, energetic people in their 30s who should be fucking like bunnies,” says sex therapist Isadora Alman. “They’ve got a roof over their head. They don’t have any major problems in their lives; but they’re just not interested.”

So clearly pornography and Hollywood rom-coms bear little resemblance to real life. Laumann once surveyed rates of sexual dysfunction worldwide.

For men, erectile problems easily topped this list; they wanted to have sex but physically couldn’t.

For women, however, it went deeper: They had a basic lack of desire.

“But given the threat sex has historically posed to women’s survival,” he adds, “I consider the latter less a dysfunction than an adaptation.”

For millennia, sex was dangerous; pregnancy could be deadly, explains Laumann. Without a committed mate to offer manly protection and sustenance, a woman’s life, not to mention her embryo’s, was a crapshoot.

Half a century ago, birth control pills made relatively worry-free sex possible, “but our evolutionary nature doesn’t change very quickly,” he says.

Unlike, say, a foal that can scramble to its feet shortly after birth, human babies are helpless, and children take years to become self-sufficient. Mothers, in short, are busy.

Helen Fisher, Ph.D., a biological anthropologist and senior research fellow at the Kinsey Institute, says humans have a dual reproductive strategy.

“We have a tremendous drive to pair up and rear our children as a team,” Fisher says, “but we also have a tendency to look for what Lord Byron called ‘fresh features.’ Not everyone cheats, of course, but most of us have some predisposition toward this.”

Our sagging sex life had plenty of possible causes.

Maternity and breastfeeding seemed to limit Debbie’s libido; my testosterone waned as I transitioned from procreator to provider. We’d both gained pregnancy pounds—Debbie’s earned, mine by proxy. And we both began to snore.

The Darth Vader vibe of my CPAP machine did not turn Debbie on. We began sleeping apart. We engaged in sex on a quasi-regular basis, with notably less of the frequency and abandon of our early years.

In a groundbreaking 1998 paper in Human Nature, Fisher outlined the three stages of pairing off: lust, attraction, and finally attachment. Each of these “systems” is accompanied by its own array of neuro chemicals.

Let’s start with lust, which is linked to testosterone. Testosterone levels are much lower in women, who do seem less easily addled by lust.

The attraction system—call it romantic love, passion, infatuation—makes men and women literally lovesick: depressed, anxious, euphoric, despairing, manic.

Our third system, attachment, is also called companionate love. It’s that deep sense of security, social comfort, trust, and safe harbor you have with your chosen partner.

The key brain chemicals here, says Fisher, are vasopressin and oxytocin, the so-called cuddle hormone, both of which play pivotal roles in attachment.

These roles vary depending on a person’s genetic makeup. An individual’s response to oxytocin and vasopressin influences marital stability and discord.

“That early stage of intense, romantic love drives up the dopamine system, which in turn increases testosterone and strong libido,” says Fisher, author of Anatomy of Love: A Natural History of Mating, Marriage, and Why We Stray. “But over time, attachment becomes more and more powerful, and this does not necessarily trigger the sex drive.”

Explains a lot, right?

“Maybe this is desirable,” suggests Susan Whitbourne, Ph.D., a professor of psychology at the University of Massachusetts at Amherst. “If we spent our days in the throes of all-consuming love fires, we’d never get anything accomplished.”

What does all this have to do with the frequency of sex? A lot, actually.

There’s more to sex than procreation and pleasure, say evolutionary psychologists at the University of Texas at Austin.

During a casual fling, for instance, some people say they’re driven to improve their skills in hopes of turning it into a long-term commitment.

Related:  In a relationship, women and men say sex fosters emotional closeness and mutual affection, helping solidify their commitment.

Less lovey-dovey facts: A woman may initiate sex to keep her husband satisfied (and deplete his sperm count), preventing him from impregnating rivals. A man may subconsciously do something similar—overwhelming any rival’s semen with frequent copulation.

Both sexes report a host of other less-than-lofty motives for having sex—an affair to punish a partner, or for money, social status, or job promotion.

Given all this, sexual frequency isn’t necessarily a good barometer of relationship health, says UT Austin’s Daniel Conroy-Beam, Ph.D.(c).

“Human long-term mating is about much more than just sex, and our motives for having sex are not always good,” he says.

“Sometimes we have sex not because we want to but out of obligation, duty, or fear of losing our partner. If the decline in sexual frequency within a marriage is driven by a decline in these more negative emotions, it’s possible this might even be a good thing for the state of the American marriage.”

So what’s ailing American sex lives? Let’s have a look.

Sex-Life Killer: Your Past

Walking around with our Pleistocene-epoch genes can be tough, especially on a college campus.

“The average college freshman sees more attractive females in a single day than our hominid ancestors saw in an entire lifetime,” says UT Austin psychology professor David Buss, Ph.D.

Combine this with social media and dating apps, and mate choice seems limitless.

Willoughby, an assistant professor at Brigham Young University’s School of Family Life, says today’s 20-somethings are showing anxiety, “terrified about making a mistake. There’s this pressure to pick the perfect person who will make them happy and fulfilled for the remainder of their lives.”

So sow your wild oats and get it out of your system, right? Maybe not.

Willoughby’s research has found that the more premarriage partners people have, the lower the sexual quality, communication, and relationship stability is during marriage.

Possible reasons: The more relationships you’ve had, the easier it is to cut and run; skills like communication and compromise aren’t developed.

This can lead to the “comparison effect.” If you were once a player, “it’s easy to compare in your mind all these previous experiences you’ve had,” he says.

This sense of missing out can erode sexual satisfaction with your long-term partner. Plus, we’re living unimaginably longer than our ancestors did.

“Even 200 years ago,” Buehler says, “people married young, had kids young, and were dead by the time they hit 40. Today, we’re outliving the natural life of our hormones.”

Modern men can procreate decades longer. Or pretend they can.

Which brings us to the cast of thousands of imaginary partners in today’s pornography. This may affect marital relations—a bit.

“We do have enough research now to suggest a weak negative relationship between viewing pornography and relational and marital sexual satisfaction,” Willoughby says. “It’s not strong, but it’s there.”

The negative tug, so to speak: It’s about expectations.

The porn star is “willing to do anything and everything the male partner wants her to, and taking great pleasure in doing so,” Willoughby says. “After watching all these clips, he starts thinking, ‘Gosh, why is my wife not in the mood? Why is she saying she’s too tired or she had a long day?’”

Sex-Life Killer: Your Kids

“Kids are the most effective libido squashers I know of,” says Alman.

Wee ones have a tendency to hang on to their caregivers like monkeys, providing so much physical touch that the last thing you want is more groping from a partner.

Touch, notes Fisher, releases oxytocin, further bonding parent to child while temporarily suppressing dopamine and libido. Breastfeeding and general exhaustion can further deplete desire.

In one study, Laumann surveyed women in their 20s about their desire for sex.

In those without children under six, 34 percent reported no interest; in those with kids, the number soared to more than 95 percent.

A man may find himself at the end of the queue for affection. It’s easy for him to feel unappreciated and even a smidge resentful, says Alman.

Buehler says it’s not surprising “that couples with children under age 5 have the least sex and report more sexual dissatisfaction than any other group.”

The recent trend toward delaying pregnancy may further exacerbate all this, Buehler says—obviously, parents in their 30s and 40s are not as energetic as they once were.

Sex-Life Killer: Stress

If both partners work, finding time to be intimate can be hard.

In a time crunch, sex may not be a priority, a study in the Journal of Marriage and Family reports. Different shifts, child care, aging parents: These stressors can trigger a cascading hormonal response that can affect libido.

But even couples who aren’t growing to resent each other can gravitate toward sexlessness.

“Sex can be a lot of work,” Alman says. “The woman may feel she has to shave her legs, she may need a long time to orgasm—there’s a lot of stuff involved, and sometimes the payoff just isn’t worth it.

Sometimes she thinks, ‘My vibrator can get me off a lot quicker,’ and he thinks, ‘I can sit and watch porn and get just as hot and feel just as satisfied.’”

Another huge bedroom buzzkill: A whopping 11 percent of Americans take antidepressant medications.

Alas, research shows that these drugs can cause and worsen many forms of sexual dysfunction, from fading libido to the inability to climax to “emotional blunting.”

Though these medications can smooth out the emotional lows, they also seem to cap the highs, putting the brakes on sexual excitement, passion, and maybe even love.

Sex-Life Killer: Comparisons

If you’re thinking about leaving this article on your wife’s nightstand, hold off.

Take a moment: Is anything truly broken?

If both of you are okay with your sexual frequency, be it nonstop, middling, low, or none, then from Alman’s point of view there really is no problem.

“If you’re happy and your partner is happy, those are the only votes that count,” she says.

Yeah, right. In our sexualized culture, it’s easy to think you’re pathological or at least an oddball.

“The reality is that more couples live happy lives, even with no sex between them, than most people would imagine,” adds Alman.

Even sexually active couples should resist the urge to compare. When researchers at the University of Colorado asked more than 15,000 people about their sex lives, they did find a link between sexual frequency and happiness.

But that happiness was relative: If people knew their peers were having more sex than they were, their happiness dipped.

“Many people just assume that everyone else is having fantastic sex five nights a week while they’re lucky to get it on their birthday,” says Alman. “A lot of what I do as a therapist is letting couples know what’s what: No, not everyone is having better sex than you are. No, not everyone has a bigger penis. People can be really adept at making themselves unnecessarily miserable.”

Sex-Life Killer: Mismatches

Discord often has less to do with frequency than with a discrepancy between how often each partner wants it. A partner who is feeling sex-deprived can wonder if a mate’s lack of interest is evidence that the love is gone.

Both partners should acknowledge that dry spells happen.

“It is completely normal for a couple’s sex life to have peaks and troughs,” says Buehler. “The important thing is to discuss the troughs. Do you both understand why sexual frequency has slid—the birth of a child, perhaps, or the illness of a parent? If so, accept it and make a pledge to get back on track when the period of extra strain has passed.”

Clients roll their eyes at one of Buehler’s suggestions for kickstarting sex: scheduling it. “They resist the hell out of doing this because they want to be ’spontaneous.’ I say good luck with that.”

Tamar Krishnamurti, Ph.D., of Carnegie Mellon University, adds a cautionary codicil to such counsel, which has become a staple of sex therapy.

In a 2015 study, she and colleagues at Carnegie Mellon split 128 married men and women into two groups and told one group to double their weekly frequency.

Not only did doubling sex fail to make affected couples happier, it led to a small decline in their happiness. They also reported a decline in both sexual desire and enjoyment.

When sex becomes a homework assignment, it can quickly lose its luster—a phenomenon that’s well documented in infertile couples who are forced into sex-on-demand dictated by the ovulatory cycle.

It’s better to plan to share pleasurable experiences with your partner without necessarily making intercourse the goal.

“Our desire to initiate sex itself diminishes more quickly than our capacity for pleasure,” Krishnamurti explains. “Focusing on creating pleasurable experiences may allow an increase in sexual intercourse frequency to happen more naturally.”

Alman adds: “Sex doesn’t always have to equal penis-in-vagina intercourse. Cuddling, kissing, rubbing against each other in ways that are pleasurable and can result in orgasm to either or both, or maybe no orgasm but certainly pleasure. Aren’t these sex too? In my book they are.” And don’t discount the power of affectionate touch.

One study found that the more cuddling, kissing on the lips, and hugging couples engaged in, the more easily they were able to resolve their conflicts.

Perhaps the toughest dilemma for couples is when one partner decides the thrill is gone and won’t be coming back.

“One perspective holds that when you’re hot you’re hot, and when you’re not you’re not, and nothing can change that,” Alman says.

“The other holds that you can learn to love your partner again by focusing on what is lovable about him or her, what originally turned you on, or what might be changed that could reawaken love and desire. As a therapist, I’m of the opinion that the second approach is certainly worth a try.”

Despite all the depressing research, most couples have some kind of sex well into old age. Our brains are optimized for love, not just passion.

“As you uncover anew your original feelings of why you were attracted, some of the humdrum may fade in the process,” Alman says. “Falling in love, even into ‘like,’ is a delightful feeling, all the more so if it’s with your own mate.”

“Well, there’s always stuffing,” Debbie joked in bed.

This is an ancient ED remedy practiced by tantric yogis; she found it in a yard-sale paperback. Any couple, the book claimed, can achieve union via the patient packing of a flaccid lingam [penis] into an artificially lubricated yoni [you know]. We’d always joked about stuffing, never imagining it might one day prove, well, useful.

It’s been many, many years since I first spied Debbie in that yellow dress.

I’m certain we aren’t the only long-term couple for whom the metaphorical bean jar remains far from empty. But the calculus here, which in youth seemed so depressing, no longer strikes either of us this way. If reverse cowgirling must someday give way to stuffing, so be it.

Love, not sex, is the only thing we can no longer live without.