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

• Once you enter a serious relationship, sex can get complicated.
• We asked Rachel Sussman, a NYC-based marriage counselor, about the most common problem couples have in the bedroom.
• Those problems include sexlessness and one partner being resistant to the others' fantasies.

Sometimes, getting into a serious relationship means that sex becomes less, well, sexy. Both people are busy and there's no time to do it. One person would rather have a glass of wine and watch "This Is Us." It's gotten boring.

These aren't reasons to be ashamed — you're hardly alone in your plight and there are plenty of potential solutions out there.

I recently spoke to Rachel Sussman, a relationship expert and marriage counselor in New York City, about the most common problems couples encounter related to physical intimacy, and her typical approach to helping them. Read on to see what's really going down in your friends' bedrooms.

SEE ALSO: A relationship therapist breaks down the 10 most common fights couples have

Partners have mismatched sex drives

Sussman said this is the most common problem she sees related to physical intimacy. Typically, one person wants to have sex more often than the other, who's either happy with the amount of sex they're having or wants even less.

Sussman usually takes a two-pronged approach. She'll work with the person whose sex drive is lower to see if there's anything they can do to increase it. She'll also work with the person whose sex drive is higher to be patient with their partner and to manage their expectations around sex.

Sometimes "the person with the higher sex drive takes on a predator-like role and that's not healthy," Sussman said.

She might even give the couple "exercises": For example, they have to try snuggling and the partner with the higher sex drive has to resist the urge to initiate sex.

The couple isn't having sex at all

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.

The relationship isn't as passionate as it used to be

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.