Ian Kerner is a licensed couples therapist, writer and contributor on the topic of sex for CNN. For more on "Sex and Love Around the World," watch Christiane Amanpour's new series on Saturdays at 10 p.m. ET/PT beginning March 17.

(CNN) Today, sex seems more available to us than ever before. With just a swipe on their phones, singles can schedule their next hookup, while committed couples have an apparent "sure thing" every night. This is also known as "the marriage advantage."

Why, then, are Americans having less sex than they were 20 years ago, not more?

According to a 2017 study published in the Archives of Sexual Behavior, the large general social survey (GSS) found that American adults had sex about nine fewer times per year in the early 2010s than they did in the late 1990s, a decline that wasn't explained by longer work hours or increased use of pornography.

The GSS is a project of the independent research organization NORC at the University of Chicago, with principal funding from the National Science Foundation.

Although it's not entirely clear what is behind this dip, theories about the potential causes abound. For instance, people are having children later in life, which may make them too tired for sex.

The parenting effect

"A lot of parents feel like they've already done about 50 things they didn't want to do that day, like getting up at dawn, dealing with their child's tantrums. Adding sex to the menu just seems like too much," said Samantha Lutz, a psychologist. "So we turn to things like Netflix to unwind, which leads to immediate gratification with zero energy expended."

Parents also seem to be much more involved with their children's lives than in the past. "Parents are constantly driving their children to school and extracurricular activities," said Amanda Pasciucco, a licensed marriage and family therapist. In previous years, children had more freedom and fewer organized activities, which meant more free time for their parents.

Eric Marlowe Garrison, a certified sexuality counselor, agreed. "We're seeing more helicopter parenting, which is zapping energy that could go toward sex and other sensual activities."

But has the marriage advantage become a disadvantage?

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Sex seems to be especially good for a woman's heart -- the physical one, that is. A recent study found that women who said they had frequent, extremely satisfying sex had a lower risk of hypertension, a common precursor to heart disease. Hide Caption 3 of 10 Photos: 10 reasons to have sex tonight Regular, frequent sex may lower the risk of prostate cancer, according to research presented to the American Urological Association. A study of 32,000 men over 18 years found that men who ejaculate at least monthly may be less likely to be diagnosed with prostate cancer later in life. Hide Caption 4 of 10 Photos: 10 reasons to have sex tonight You won't look like this without hitting the gym, of course, but sex is a form of exercise, burning about 150 calories an hour . Add it to the end of your workout as a reward with benefits. Hide Caption 5 of 10 Photos: 10 reasons to have sex tonight Sex can lessen pain. Studies have found that even stimulation without orgasm can reduce menstrual cramps, chronic back and leg pain, even migraines . Something to think about the next time you consider saying "Not now, honey, I have a headache!" Hide Caption 6 of 10 Photos: 10 reasons to have sex tonight Sex may help your memory, too. Men over 50 who had more sex were better at word recall and number sequencing, while older women improved only in word recall, according to a study published by Oxford University. Both did better than those who had less sex. Hide Caption 7 of 10 Photos: 10 reasons to have sex tonight Sex lowers stress and anxiety by releasing all sorts of good-for-you hormones, and that can help ward off depression, too. Studies show that men and women who have intercourse with their partners have greater satisfaction with their mental health. Unfortunately, the benefits didn't extend to masturbation. Hide Caption 8 of 10 Photos: 10 reasons to have sex tonight Good sex improves sleep, too. After orgasm, the hormones prolactin and serotonin are released, helping you feel relaxed and sleepy. Women ( and some studies ) argue that men receive the greater benefit. Hide Caption 9 of 10 Photos: 10 reasons to have sex tonight In case you didn't know it, sex can also make a baby. And that can be good for you. Studies show that people with kids living at home tend to have more money and are more highly educated and in better health. Hide Caption 10 of 10

"We don't know if indeed the 'marriage advantage' is now a 'disadvantage,' " said Debby Herbenick, president of the American Association of Sexuality Educators, Counselors and Therapists, "in part because so many other things have changed about sex and health in America in recent decades, including far more Americans taking medications with sexual side effects, far more Americans dealing with chronic health conditions -- like diabetes -- known to affect sex, and millions more Americans surviving cancer -- which is wonderful -- but often with sexual side effects from cancer treatments. An important question for future research would be, all else being equal (including health status), how does marital status relate to people's sex lives?"

The Tinder effect

For committed couples, technology can be a distraction that takes attention away from their partner. "Whether they're at dinner or walking their dog, I see more and more couples looking at their respective phones while they're together," said Marty Klein, author and certified sex therapist. "This reduced conversation and attention for each other can't be good, as it reduces the 'simmering' feeling that so many people want as the context for sex."

"Many couples find themselves in bed looking at their devices, freely admitting that they're doing nothing all that compelling," added Deborah Fox, a psychotherapist. "They've allowed themselves to drift into habits that disconnect them. If sex has become boring for these couples, it's because they've allowed the drift to dampen their creativity, not because that YouTube video is so much more exciting."

That might seem obvious for couples dealing with the routines that settle into long-term relationships, but with single folks, technology -- including dating apps such as Tinder -- may reduce the likelihood of sex, too. "In the age of Tinder, people are having more cybersex," Pasciucco explained.

"Sexting and sending photos is rampant," she said, "and I have heard more and more people reporting to me that they send nude photos to people they've never met -- and never intend to meet!"

Francie L. Stone, a clinical psychologist, has observed similar behaviors. "In speaking with young women and men, it seems (they use) dating and hookup apps more as a way to affirm themselves. The women in particular say that they use these apps when they are drunk or bored. They never meet up and have no intention to; they just enjoy the sexual banter. They're using it as a replacement for 'real-life' sex and to explore their own sexuality, what they may like and what they may not like, all in the safety of their own environment."

This may lead people to masturbate instead. "I think young singles having less sex boils down to them having more choice. They can be picky, and they are," said Holly Richmond, a certified sex therapist. "If it can't be a stellar experience, they'd rather not bother when they know they can go home to great porn and high-quality toys that deliver grade-A orgasms."

Herbenick, also an associate professor at Indiana University School of Public Health, added that "we may be having less sex, but I would argue it's better sex. We actually don't know if singles are having 'less sex' since the (survey) never defined sex and doesn't ask about the many kinds of sex play that people engage in (including masturbation, oral sex and sex toy play), especially during hookups. It is possible that singles are having less frequent intercourse but about the same (or more or less) of other kinds of sex, such as oral sex or hand stimulation or sex toy play."

Is less sex really a bad thing?

"As a certified sex therapist, I am more concerned with the type of sex -- is it intense on an emotional, intellectual, physical and spiritual level? -- than I am on the number of times per week," Pasciucco said.

In fact, the drop in sex might be the result of culture shifts and female empowerment, explained Margie Nichols, a psychologist.

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Compared with earlier generations, women might be viewing sex as less of a duty to their husbands and more of a personal choice. "It makes sense that women in relationships might be losing their sex drive and saying 'no' more, as opposed to my mother's generation that just spread their legs and composed a shopping list in their heads during sex," she said. "If that's true, then the decline in frequency is a good thing."

On that note, remember, a healthy sex life is whatever works for you and your partner. It can't be measured by a statistic.