If this is actually an accurate representation of the situation (the Otaku seem interested in "simple", unrealistic relationships, not necessarily uninterested in sex to me), the perspective of the female population would be very interesting. Do they share the lack of interest in sex? Are they still interested in it, but find the small remaining male population with similar interests sufficient, i.e. several of them date the same guy?
It's also good to keep in mind that sex is no longer required for reproduction, so the conclusion (dramatic population reduction until 2060) is overly dramatic.
An early guardian article [0] suggested that both men and women have moved away from relationships. In particular, women avoid a relationship because that would mean giving up their career and freedom in order to produce babies and do housework all day. It seems that Japan needs to update its expectations of a woman's role in domestic relationships before today's independent, educated women will desire stable relationships.
It's also good to keep in mind that sex is no longer required for reproduction, so the conclusion (dramatic population reduction until 2060) is overly dramatic.