A relationship therapist breaks down the 5 most common problems couples have in bed

Interestingly, Sussman said that couples in this situation "are not always upset about it." Instead, "they feel they should be doing something about it" and think, "This can't be normal."

Sussman's approach here is to "dig a little bit." She said, "I want to see what it was like when they were [first] dating. Did they both have a sex drive then? Did they have a lot of sex back then? What kind of sex did they have? What were the patterns? How did it feel?"

If it turns out the couple used to have a more active sex life, Sussman tries to figure out what's changed. It could be many things, she said: they're stressed, they've gotten too familiar with each other, they're repressing their sex drive, or they've gone so long without sex that now they're shy with each other. It's possible, too, that one person is having an affair.

In many cases, Sussman will assign the couple to have sex more often, then come back and report to her how it went. "More times than not, they'll come back and they'll say to me, 'That was a lot of fun. I don't know why we don't do that more.'"

Interestingly, Business Insider's Jessica Orwig reported that one study found couples assigned to double the amount of sex they were having didn't wind up any happier. The Carnegie Mellon researchers behind the study recommend focusing on quality, not quantity — though this strategy might not apply to couples who aren't having any sex.

Sussman's view is that the inevitable decline of passion in a romantic relationship is evolutionary. Thousands of years ago, people didn't live long enough to have to sustain passion with the same person for 50 years. Now, many of us do.

So when it comes to sustaining that passion, Sussman said, "you've got to be creative."

One strategy is to schedule "sex dates" — a tip Business Insider has heard before. That's especially helpful if you're both busy, if you have young kids, or if one partner goes to sleep earlier than the other.

"Don't feel like something's wrong with your relationship if you don't naturally have [passion]," Sussman said. It takes work to develop and maintain it.

In fact, Pamela Regan, a psychologist at California State University, Los Angeles who studies romantic relationships, previously told me that's something couples in love-based marriages can learn from couples in arranged marriages. The level of passion in a relationship may fluctuate and it doesn't necessarily mean something is wrong — as long as you have faith that it will return and the willingness to help lure it back.

In some cases, Sussman said, one person might have sexual fantasies they don't feel comfortable sharing with their partner. In other cases, they may have tried sharing a sexual fantasy, only to be told they're "perverted."

A similar issue Sussman's seen is that one partner enjoys watching pornography and the other doesn't. Or, one partner was "caught" watching pornography and the other partner doesn't feel comfortable with it.

More and more couples are coming to Sussman with this issue, she said. Usually they're trying to figure out "if it's something that they think they could even do." Or, they might want to talk about setting "rules" for pursuing an open arrangement.

Perhaps unsurprisingly, "What is an open relationship?" was one of Google's most-searched terms related to relationships in 2017.

Biological anthropologist Helen Fisher, who is also the chief scientific adviser to dating site Match, previously told Business Insider there are two kinds of open relationships: swingers and polyamorous couples. Swingers go out together and have sex with other people; polyamorous people typically have individual romantic relationships with others.

Fisher cautions that if you're thinking about initiating an open relationship, be prepared to set a lot of rules — and potentially to feel more jealous than you thought you would.

Sussman said couples who come to talk about an open relationship aren't necessarily set on having one. "Sometimes people just don't know how to even go about having the conversation," she said. "They just want to start a dialogue in a safe space."