The problem isn't that they aren't having sex, but rather than they're using birth control while doing so. Having children is a very conscious decision in this

The problem isn't that they aren't having sex, but rather than they're using birth control while doing so. Having children is a very conscious decision in this day and age, and the Japanese are on average not living in fantastic conditions for raising children. Wages which haven't risen in real terms for ~20 years, little paid maternity leave (or none?), small living spaces, and many others. Of course there are the Hikkikomori issue (young people who shut themselves in their room and never leave) and Soushoku danshi (men who take no interest in pursuing the opposite sex and forming relationships), two rather small factors reducing the birth rate further. But ultimately I think the economic issues are the biggest. Japan just isn't prosperous anymore, and people don't feel wealthy enough.