I think there is enough masculinity in Japan.
I noticed it when I visited there.
I dont think you can lump people together and say they are just one thing.
There are many aspects in Japanese culture you havent considered.
When it comes to reproduction rates, always bear in mind that the establishment of the welfare state could be mainly responsible for the drastic decline due to the separation of intergenerational responsibility.
I am pretty sure that most of the developed asian countries are having a decline in birth.
As we are becoming a developed society, some of our habits change.
Poeple dont need to reproduce as much anymore.