The title on HN is downright misleading. "... the Japan Family Planning Association interviewed 3,000 subjects, both male and female, about their sex lives." Those aren't couples, they're just random men and women from what I understand this text.

Also, I have lived in Japan for the past 5 years and around me at least a lot of people are (very) sexually active, in fact, too active, or too casual usually. Most guys date multiple girls/have multiple sex friends, and even if they're married or in a relationship, many have sex partners other than their girlfriend/spouse.

Despite the misleading title, I wouldn't be surprised if couples reply that they haven't have sex with each other for the past month and were telling the truth (because they're too busy having sex with other partners).