The reason for Japan's low fertility is its anti-male culture. Men are supposed to work 80 hours a week until they retire, at which point they're useless and should have the decency to die. Nothing is expected of women, so they spend their days sleeping around and complaining about the largely mythical train groping issue. Japanese men don't want to raise families with entitled whores, so instead they just nope out of society.

It's not really a gender issue. It's a labor expectation issue, and it affects women too. People work insane hours in Japan and the work culture is really toxic. Having children would just enslave people to more of the same, so why bother.