This study/paper actually found that part of the decline in sexual interest in the youth in Japan could be attributed to a rise in asexuality.
Have Japanese People Become Asexual? Love in Japan
"Data were collected through an internet survey; this article uses a subsample of 9,940 respondents. The dependent variables were the number of lovers respondents had during and after junior high school; the independent variable was the respondents' age. Regression analyses found a clear trend toward asexual behavior among young men (in their twenties and thirties). However, young women are not as asexual as the previous generation. This asexualization of men may have contributed to Japan's sharply declining marriage and birth rates. If so, other Asian societies may learn lessons from Japan's experiences in the future."
This kind of thing appears to be what Kojima is drawing parralels to in Death Stranding's fictional (key word fictional) world or back-story.
This honestly just seems like the author picking a really weird definition of asexuality.