Susan Heitler, a couples psychologist in Denver who also founded the online counseling resource Power of Two, said previous studies have found relationship needs change over time.
"Prior research has shown that as people age, men focus more on general relationship satisfaction and women ... increasingly value the sexual aspects of the relationship," she said.
There could be a number of reasons why women's need for sexual satisfaction increases over the course of a relationship.
"Maybe after 15 years of being married, it's not a difficult time as far as raising children is concerned, and women can devote more time to their sexual satisfaction," she said.
Experts also say biology could also be a reason.
"Men are at their sexual drive peak in their late teens while women are at their peaks in their early 30s," said Fran Walfish, a Beverly Hills, Calif.-based psychotherapist and author of "The Self-Aware Parent."
"Men may become less concerned about sex because they are feeling spontaneously aroused less frequently with age," said Heitler. "They still like sex, but they feel less desperate for it than in their younger years."
Many women establish identities outside the home and as they get older, feel more empowered to get their sexual needs met.
"Women gain ego strength and a sense of their own sexual rights as they age and so if expectations are not met or their partner does not attempt to please them, they can be upset, even angry," said Pepper Schwartz, senior fellow at the Contemporary Council on the Family and author of Prime: Adventures and Advice about Sex, Love and the Sensual Years.
Other factors that predicted relationship happiness in the study were good health among men and being able to function well sexually among men and women.
While men and women differed about what made them happy in their relationships, both sexes reported being happier the longer they were involved with their significant others.
The study also found differences in relationship and sexual satisfaction across cultures. For example, Japanese men reported being more satisfied sexually than American men, and Japanese and Brazilian women were more satisfied sexually than American women. These data may not paint an accurate picture, however, since subjects from the different countries weren't all similar in terms of factors like age.
"The research does show interesting findings that need to be explored much further to see how it translates to a large international population," said Amy Levine, a certified sexuality educator in New York and founder of Sex Ed Solutions, a sex education web site.
This study may also offer comfort to many men who think they're the only ones whose wives or girlfriends aren't always interested in sex.
"Perhaps this will soothe some of the men in their thirties and forties who think that their situation of having a wife who is not all that enthusiastic about sex is unique," said Zoldbrod.