Have been reflecting on this issue recently.
The modern society base everything on career success, wealth, prosperity, self esteem, ego-centrism. We live in capitalism. It is obvious for many people that children are big responsibility and effort mentally, monetary and physically.
Therefore it is apparent that when you have a baby it's extremely hard in contemporary state of society to raise it properly without diminution of pleasures or even opportunities to enjoy your own life as you want it. Not even slightest idea of any dependents.
Sex is integral part of intimate relationship between healthy people. So it's a delight seeing many young Japanese people who have a very logical thinking and therefore don't start any relationship, because it is very rational decision for the reasons I provided above.
There is an awesome movie Paris, je t'aime. One of the stories from that movie is when someone called Ana leaves her child at daycare to go and babysit some rich lady's child. The scene is very sentimental and not easy to watch, but it is true. It's our reality. We leave our children to work and to earn money, because that's the only way we can survive in the current set up.
In addition to those issues nothing is being done conceptually to improve the situation. There is no stimulus to move from "the realm of necessity to the realm of freedom".
As far as freedom equals to more money on bank accounts there is no surprises that many young Japanese prefer staying without proper relationships and sex.