Sex is an important part of a relationship. It bonds couples, and it's an intimate way of expressing love and desire.

However, a serious problem for those in long-term relationships can be a lack of sex or sexual desire entirely. This even goes for relationships that began with a powerful spark.

According to research, many couples struggle with lack of sex in their marriages. Independent market research company Gitnux found that about 15% of marriages in the United States are sexless and about a third of divorces are caused by sexless marriages.

But a relationship doesn't have to stay sexless. Mental health experts say it's certainly possible to revitalize sex in any relationship, no matter how long it's been missing − but it takes effort and intentionality.

"Sexless relationships, sex ruts, mismatched libidos − I think that's the number-one problem that couples therapists deal with, especially when dealing specifically with sexuality," says Ian Kerner, a therapist and sex and relationship specialist. "I think in long-term relationships, it's very natural for this to happen. One thing that I've noticed in these relationships is that sometimes it's not even that couples want sex. What they're missing is being desired."

Why do some marriages become sexless?

The reasons why some marriages become sexless are vast and varied. They also have stirred heated debate online.

In March, for instance, a video went viral of a woman explaining why she thinks some women choose to stop having sex with their husbands. Her explanation involved attachment theory, which experts say has some validity, but is only part of the equation.

Stephanie Sarkis, a psychotherapist and author of "Healing from Toxic Relationships: 10 Essential Steps to Recover from Gaslighting, Narcissism, and Emotional Abuse," previously told USA TODAY attachment needs can certainly affect a couple's sexual intimacy but that the reasons for decreased sexual desire are often more nuanced and cannot be mainly attributed to attachment.

Some reasons are also entirely out of the couple's control.

"There could be a physical health issue. There could be a communication issue. There could be hormonal issues, which leads to lack of desire. It could be there's built-up resentment. It could be that there is a lack of connection," Sarkis said. "To say that it's solely due to attachment issues is painting too broad of a picture."

Is this the answer?This woman's take on why wives stop having sex with their husbands went viral. Is she right?

Erik Anderson, a licensed marriage and family therapist, previously told USA TODAY it's also normal for sexual desire to spike in the beginning of a relationship − often called the "honeymoon phase" − and decline as a relationship goes on. He also said it's possible for people to struggle to feel sexual desire in their relationships even while feeling emotionally connected to their partner and that there can be a plethora of reasons for this, ranging from biological issues to unresolved trauma.

"Sometimes (sex) just withers away because of these things that we don't address with ourselves that really don't have much to do with attachment in the relationship and emotional vulnerability with one another," he previously told USA TODAY.

Sex with a narcissist can be electric.It makes relationships with them more confusing.

How to bring sex back into your relationship

Sex, however, can be brought back into any relationship with the right planning and effort.

According to Kerner, most often, couples that have lost their sexual chemistry have lost something he calls the "erotic thread." He describes this as "the flirting, the sexting, the grabbing, the groping, the teasing, the touching" and other behaviors that cultivate desire and build anticipation for sex.

Setting an intention to bring these components back into your relationship can make a huge difference in boosting your sexual chemistry, he says. For couples struggling with sex, he recommends picking a date in advance to have sex and working on building the erotic thread in the days leading up to it.

"Know that there isn't a demand to have sex right now," Kerner adds. "Just focus on the giving and the receiving of that erotic energy."

More:People are paying thousands for 'dating boot camp' with sex experts. I signed up.

Sometimes, sexual connection also gets lost due to a couple's limiting, false beliefs about sex, Anderson says. Identifying and shifting these attitudes can make you and your partner see sex in a different way.

"People make the assumption that sex is for young people," he says. "Making an explicit statement to myself and my partner, 'I want the best sex of our lives to be ahead of us,' can be really helpful in realizing that that's possible."

When you and your partner do start having sex again, Sarkis says, it's important to keep your expectations realistic. Sex between you and your partner is probably going to be very different than it was when you first started having sex − and that's OK.

Being present with yourself and your partner as you both currently are can make you more connected.

"Be aware that it's going to be different than when you first met, because people change, but you also have a deeper connection," she says. "You're not trying to recreate what you had the first year that you were together. You're trying to find a deeper connection."