A young newlywed recently confided to me that he’s worried about his sex life: “I know we’re supposed to do it at least every other day,” he said. “But we can’t keep up.”

Who wants to tell him?

Some of the most commonly held beliefs about sex are incorrect, according to a growing body of research. Recent studies reveal new insights about desire—how to spark and maintain it. Others are overturning long-held ideas about the ideal frequency for physical intimacy.

Misconceptions about sex are often passed along between friends, parroted in chat rooms or spread by uninformed social-media influencers. Even well-meaning doctors and therapists who lack specific training in sexual issues sometimes pass along misleading advice.

“We have a sexual literacy problem in our culture,” says Justin Garcia , executive director of the Kinsey Institute at Indiana University. “And in the absence of good information, we create a mythology and treat it as gospel.”

We shouldn’t be surprised. Sex education and research are perpetually underfunded and often under attack from politicians and religious groups, Garcia says.

Our own reticence plays a role, too. Many people are uncomfortable talking about sex.

Yet good relationships benefit from good information. With that in mind, here are several commonly held beliefs about sex that recent research has debunked.

You should be having more sex

Many people believe that happy couples have a lot of sex—and that those who don’t are headed for relationship trouble.

“No matter how much sex we are having, we think we should be having more,” says Gurit Birnbaum , a professor of psychology at Reichman University in Israel, who studies sex and relationships.

That notion is false!

Different couples have different sexual needs. Some prefer a lot; others, less. A couple’s ideal frequency also changes over time, with age, length of relationship, illness and stress all having an impact.

Still, there’s a sweet spot: People who have sex an average of once a week report greater relationship and life satisfaction than those who have it less frequently, according to research included in a review of 279 studies on sexuality, published earlier this year in Nature Reviews Psychology and co-written by Birnbaum. And—surprisingly—those who have sex more than that don’t report being any happier.

This might be because once a week is enough to maintain connection, the researchers say. More might become routine—and exhausting. Also, the idea of a sexual afterglow is real: People remain more satisfied with their relationship for days after they have sex, research shows.

“You don’t have to have sex every day to feel the intimacy and the closeness,” Birnbaum says.

Keep fantasies to yourself

Almost everyone—97% of the population—reports having sexual fantasies, according to Justin Lehmiller , a senior research fellow at the Kinsey Institute who, since 2014, has been conducting one of the most comprehensive studies ever done on fantasies . Yet therapists, he says, sometimes caution people against sharing them with a partner, warning that it could create unnecessary conflict or even doom the relationship.

Lehmiller’s research shows that the opposite is true: Most people report positive experiences when they share their fantasies—even when the fantasy is about opening up a monogamous relationship to other people. (After all, fantasies don’t have to be acted upon!) Sharing can bring partners closer, create more excitement in the bedroom, and lead to greater relationship and sexual satisfaction, he says.

Deciding whether to share depends on the partner, the relationship and the fantasy, Lehmiller says. To determine what’s right for you, he suggests asking why you want to share. Is it to jazz things up in the bedroom, help your partner understand you better, or act out your fantasy? “It will help you have clarity and present it to your partner in the right way,” he says.

And remember: You don’t have to share every fantasy you have, especially if you think it might be a turn off or make your partner feel insecure.

“Let’s say you have a fantasy about your partner’s best friend,” Lehmiller says. “You may want to keep that to yourself.”

The best sex is spontaneous

Watch any movie with a sex scene and you’ll likely see two people, overcome with lust, ripping each other’s clothes off. No wonder most people believe that the most passionate, satisfying sex just happens.

And perhaps it does if you’re newly in love or you don’t have a job, mortgage or kids. For the rest of us, though, it takes a little planning.

Here’s the good news: People who plan for sex—think date nights—find it just as good as spontaneous sex, research shows. And one soon-to-be published study finds that it’s sometimes even better: When participants were told about the benefits of scheduling sex, they had it more often and enjoyed it more.

“Planning can build anticipation,” says Amy Muise , a professor of psychology at York University in Toronto and director of the Sexual Health and Relationships Lab, who co-wrote the research. “And it shows that it’s a priority.”

After all, most of the important things in our life are planned.

To make scheduling sex seem less mundane, try thinking of your sessions as trysts, says Laurie Mintz , a sex therapist and emeritus professor of psychology at the University of Florida. She often advises clients to picture their early dates with their partner—the way they put on perfume and sexy underwear, flirted shamelessly all night and had sex at the end of the evening.

“That was never unplanned,” she says. “It was so well-orchestrated that you just tricked yourself into believing that it was spontaneous.”

Write to Elizabeth Bernstein at Elizabeth.Bernstein@wsj.com