By which we learn that marriage per se is not beneficial for sexual satisfaction

This paper is not good news for governments of countries that face a low birth rate and that usually for economic reasons.

Does Marriage Really Improve Sexual Satisfaction?

"Previous studies have argued that marriage provides a better ecosystem for sexual satisfaction and that married couples are more sexually satisfied than singles are.. Others even argue that believing in marital sex as part of a religious system increases sexual satisfaction... The arguments that married couples enjoy a higher level of sexual satisfaction stand out against the background of a growing number of individuals choosing to meet their sexual desires outside of marriage. ... 

Using the ninth wave of the Pairfam data set and analyzing the responses of 3,207 respondents in total, this study suggests that marriage is not a determinant for sexual satisfaction. In fact, it can even be a negative correlate when married respondents are compared to certain unmarried groups. The only exception is that of unmarried individuals who currently have no partner. Even this situation is shown to be dependent only on less frequent intercourse, not on a lack of sexual self-esteem and sexual communication.

The findings show that married people reported lower rates of sexual self-esteem and sexual communication skills than most groups. In terms of sexual communication, only never-married cohabiting people were comparable to married people for both genders, while never-married single and living apart together women and divorced separated and single women were also comparable to married women... 

The detailed and rich data of the Pairfam survey show that even singles who have never been married are more likely to report higher levels of sexual self-esteem and sexual communication and that it is mostly the availability of sex that makes them less satisfied sexually...

Moreover, the findings indicate that marriage per se is not beneficial for sexual satisfaction. In fact, married couples score relatively low in this regard. Non-partnered singles scored lower than the married group in terms of sexual satisfaction, but the main reason that they were less sexually satisfied than married couples was sex frequency, which was naturally lower for non-partnered singles than for couples. Therefore, it seems that it is not marriage that is beneficial to sexual satisfaction but rather having a partner..."

People with lower self-esteem and sexual communication marry.

"One possibility is that sexual self-esteem and sexual communication levels work in reverse: People with lower sexual self-esteem and sexual communication are more likely to marry. They might want to feel safer and cover for their lower levels in these two realms and, thus, are self-selected into marriage."

Tell her/him it is not her/his fault, just a diminishing marginal effect.

"The second possibility is that the length of their relationship emphasizes a diminishing marginal effect, as several longitudinal studies show in researching other effects of marriage over time."