I disagree that feminism can't explain the same problem in Japan, even when that society is more patriarchal. The explanation for the decrease in the fertility rate is essentially the same. Lots of women are entering the work force and having careers, while men are left to play second fiddle and lose the supporter and mother of their children they need to have a family. In japan, this process has been somewhat accelerated due to the fact that it is so patriarchal, and being a wife means have no career whatsoever.
Myself, I'm not that concerned with the drop in the birth rate.