Japan Family Planning Association researches the reason of sexless among sexless married couples in Japan. According to it, the biggest reason is "tired at work". The breakdown of the reason is below.
Figure : Reason of Sexless among Sexless Married Couples in Japan
|Reason||Men %||Women %||Total %|
|Tired at work *1||28.2||19.3||37.9|
|No reason, after having a baby *2||17.9||20.5||28.2|
|Can't be bothered *3||12.0||23.5||23.8|
|House is too narrow to keep the privacy||6.0||1.8||6.9|
|Feel like incest taboo *4||2.6||4.2||4.7|
|Prefer other interesting things/hobbies||1.7||4.8||4.1|
|Afraid of erectile dysfunction||3.4||0.0||3.4|
|Unpleased with selfish sex by the partner||0.9||1.2||1.5|
|Having physical pain on sex||0.0||1.2||0.6|
*1 : very tired at work and does not have enough energy to have a sex.
*2 : When the wife was pregnant, sex was suspended. And after the baby delivery, the interruption continues without a particular reason.
*3 : They do not feel like having a sex. It is a hassle.
*4 : Most of people do not have sex with parents or sisters or brothers or children. After the long term relationship, the partner became a such kind of family member.
Source is Japan Family Planning Association.
I cannot understand the option "no partner". A couple does not have a partner? Do they live separately?