I think cultural mores about sex may have varied throughout Japanese history. I think prior to the late 60s-early 70s they were pretty conservative. Non-virgin women couldn't get married. Even now you'll still sometimes hear women say the old phrase, "Now I'll never be a bride" that originates from those times.
You also have to take into account western influence. Japan, prior to the Meiji Era, was a very sexually liberal place; adultery by men and women was normalized and tolerated, as long as it was kept private, Buddhist priest regularly had sex with the little boys sent to stay at their temples, prostitution was common and widespread, homosexuality was not seen as sinful, Samurai engaged in homosexual relations with their pupils, sex slavery was legal, and even Miko (Shinto priestesses) engaged in a form of sacred temple prostitution.
Then, Commodore Perry opened Japan up, the Meiji Restoration happened and all these western ideas began to filtrate into Japanese society. As a result, Japan westernized itself to be like the western powers, adopting things like their clothes, government system, and culture, and as such, much of Japan's more open toleration of sexuality was ended, as the Japanese embraced Christianized sexual norms, while not actually embracing the Christianity. Then, after the second worldwide unpleasantness, America occupied Japan, and also influenced the Japanese state. America setup a very conservative Japanese government, which has more or less remained in power. Japanese censorship laws and anti-prostitution laws, for example, date back to this post-war era.
If anything, the looser Japanese moors of today are just the Japanese moving back to what was historically normal for them. Also that phrase, "Now I'll never be a bride", is probably not something you'll hear many young Japanese women from the city say; maybe country girls.
The herbivore men isn't entirely their fault. The economy sucks now and young men can't get the kind of stable and lucrative jobs like their fathers and grandfathers had. Thus, women have absolutely no interest in these guys. These men have just given up. I think it's similar to a lot of those MGTOW guys.
That's part of the reason. There are many reasons though.
This article, written by a Japanese person, will give some insight. Nobita of Japan also did a video on the subject recently here (be sure to turn on closed captions for subtitles):
If you want more information on general Japanese sexual morality see
this article,
this article, and
this article.
