Sexless marriages, though rarely discussed openly, are more common than you might expect.

Google searches for "sexless marriage" are three and a half times more common than "unhappy marriage" and eight times more common than "loveless marriage," making it the most-searched marriage complaint, data scientist Seth Stephens-Davidowitz reported in 2015.

So what exactly constitutes a sexless marriage? There's no precise definition, though some experts say it's when a couple has sex fewer than ten times in one year. For San Francisco-based sex therapist and relationship coach Danielle Harel, it's any marriage in which the sex is "mostly non-existent between the couple."

"It could be that they haven't had sex in five years, or only have it every couple of months," she said.

While a lack of sex may strain many relationships, is it possible for some marriages to stay intact with little to no physical intimacy?

According to a 2015 Pew survey, 61% of U.S. married adults say a satisfying sexual relationship is "significant" to a successful marriage.

"Most people attribute significant meaning to sex and derive relational value from it: feelings of love, attractiveness, eroticism, desire, and an expression of passion that you do not get to access with others," said Irene Fehr, a sex and intimacy coach outside of Denver. "People enter romantic relationships because they want to share sexual desire, attraction, passion, and connection. This is what differentiates a romantic relationship from a friendship."

But not everyone in a sexless marriage is miserable and doomed for divorce.

"A marriage can last long term without sex if both people are not bothered by the lack of sex in their lives," said sex therapist Celeste Hirschman, co-author of "Making Love Real: The Intelligent Couple's Guide to Lasting Intimacy and Passion." "For some people, sex is not a particularly high priority. For others, it is quite high, just like any other activity."

Under certain circumstances, a sexless or low-sex relationship may be tenable or even fulfilling, including when "Both partners have low libidos", "One partner has a health issue that affects their sex drive or performance", and "The couple values other aspects of their life together more than they value their sex life". But for many couples, a lack of sex can make it difficult to sustain the marriage.