Japan is a very sexually repressed culture and has been for quite a while. This has filtered down to the younger generation as non-interest in sex.
There are many reasons that Japan is in the situation they are in, not coming from a Judeo Christian background is not one of them (nor is their version of Buddhism responsible either).
Their corporate culture, gender roles in society (for career women getting pregnant is a career ender there, and marriage puts you at risk for pregnancy so career minded women have an aversion to sex and marriage), and their sexual repression in their public facing lives are all much more responsible.
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