I've spent a lot of time studying Japanese history, up through the American Invasion, and i ascribe the modern sexual culture and sub-cultures there down to the clash of two completely different cultures at work simultaneously in the nation. These are my observations and opinions, and should not be construed as academic fact, though in my opinion, they are likely more accurate than the opinions of the academics.
The historic Aristocratic Shinto feudal order which existed through the early Edo period reserved sex for the purpose of procreation. This did not mean sex out of wedlock didn't happen, but it was culturally frowned upon, and there was only a specific set of circumstances where it could be viewed as honorable.
These set of circumstances were beyond the means of any feudal peasants, and as such was limited to the domain of the samurai and noble classes.
Japan at the time also had the distinction of sub-cultures, where duty was considered honorable, even if the act taken in the performance of that duty was not.
An example of this which is familiar to western minds is the popularized story of The Forty-seven Rōnin.
The actions they took were completely dishonorable, becoming drunks, taking work as base peasants, murdering an aristocrat... The fact they they did it to preserve the honor of their slain lord made all the dishonorable acts honorable, and they were allowed to die as samurai.
This concept of performing dishonorable acts out of duty was not limited to this example. Take their Geisha.
Prostitution, as a concept, was outright illegal; punishable by the swift stroke of a sword, when found... At least for the peasants.
The Geisha on the other hand, was an entertainer, not a prostitute... The distinction of being hired to have sex with someone on a nightly basis notwithstanding, they were exceptionally well trained in dance, music, theater, and social graces.
Often as not, their services would be limited to the role of social hostess and entertainer, but when deemed necessary, they had the leeway to discretely service their host in the privacy of their chamber. This was considered an act of duty, to relieve the stress of the host after having hosted a gathering... Bear in mind, any social gathering in the Japanese feudal period could result in the loss of heads for the even the mildest perceived slight or tinged word.
It was the responsibility of the Geisha, or other hostess, to make sure that the attending lords were handled, and the environment managed to keep their attentions away from their honor, and on having a good time.
This was their duty. Sex is an amazing stress reliever, and properly used and deployed, an extremely powerful weapon against men in all their aspects... Including honor, war, and attention spans.
With the demise of the samurai and the rise of the industrialist businessman, they simply stepped into the role of the Samurai in the Kōka period...
They simply absorbed the same privileges the samurai had, without any of the duty... For the duty, they substituted their work ethic.
Short of showing up to impregnate their wife, (this was a duty under Shinto, you see), they donated the entirety of their lives to the business. The business became the new duty.
As a side effect of only showing up at home once a month, if that often, they were afforded the privilege of geisha. Their meetings would often occur in a geisha house, or with geisha in attendance, to help relive the stress associated with living at work.
This arrangement persisted for perhaps another 100 years or so, until the industrial military complex which led Japan on the path of world domination was unceremoniously dumped and destroyed by the American invasion.
Japan always was, and always will be an extremely well-ordered, well-mannered and well-disciplined society. When the Emperor, (a living God, in the eyes of the Japanese), declared that they must "endure the unendurable", this was carte-blanche license to do whatever must be done to preserve the notion of Japan.
The Japanese accepted the new cultural changes out of duty to the Emperors command, and out of these conflicting cultures, many nuanced sexual idiosyncrasies which are unique to Japan followed.
The westerners viewed the geisha as prostitutes, and implemented laws against them, like they would any normal prostitute. But 2000 years of history and culture is not so easily undone...
Absorbing this culture, the geisha went underground for the sex, and public for the entertainment. They became 'Idols', young girls who publicly performed, and were privately for hire to the elite few who could afford them.
The historically stylized notion of sex for procreation, external to the dutiful sex within the confines of work ethic endured, for a period.... Then came the 60's, and the 'sexual revolution' in the west.
The notion of casual sex for the common man, for the purposes of pleasure, was the single most devastating blow the Japanese ever suffered.... Far greater than the fire bombings or atomics of the war.
It has not recovered, and indeed it remains to be seen whether the Japanese, as a people will survive it...
That they are now accepting sexual deviance as a duty, has facilitated the acceptance of many sub-cultures, essentially deigning them with the honorific of 'geisha'... "Because it is art, it is my duty", lending itself to both adult gaming communities and the 'furry' and cosplay phenomenons... You will see this 'duty' rearing its heads at odd points.
As an example, the Otaku culture parallels the Miko in Shinto... Devotion to their order to the exclusion of social and familial involvement. The loss of this duty to the family, in particular the notion that it is one's duty to bring forth children to increase the power of the empire, manifests in the 'Waifu"... To which they are fanatically.... and exclusively... devoted.
This quasi-religious fanatical devotion to sexual fetishes has not served the nation well.
Japan is now a dying nation. The elder and middle aged concepts of fanatical devotion to work with children limited to the wife, has led to men eschewing marriage until they are older, and their employment and associated self-worth more established. Staunch social orders still in place since the Kankō period reflect poorly on anyone taking a bride outside of their age range... Thus, no children from this very large segment of the population.
The younger segments, rebelling against this order of 'all-work, no play', are taking it to the extreme, in many cases, and completely eschewing work... But often not to the ideal of having sex out of wedlock, which still carries the social stigma.
Instead, we find the Otaku's devoting their lives... and wives... to video games, killing reproductivity out almost entirely amongst them.
The very young and attractive girls, are identified at a young age by the corporations, and are taken on the 'Idol' path... The contracts they sign enforce celibacy, and they are forbidden even private association with males for the duration of their contracts.
This phenomenon is exacerbated by modern technology and the Asian embrace of it...
An 'Idol' no longer need have any skill at all, simply to look attractive and gyrate in a suggestive manner... The music, lyrics and singing voices for the albums are provided for them, via proxy or digital manipulation.
Their birth rate has exceeded international declines among western nations, as these factors combine with the western popular myth of overpopulation endlessly repeated and venerated by the leftist media, to Japan having a current birthrate of only 1.46... With the minimum sustainable rate being 2.1... The rate to replace both parents with nominal growth.
The very right-minded feelings of the Japanese, preventing immigration to preserve their being "The Japanese", is a small factor in these matters. Though the leftist media would portray otherwise, the fact remains that Japan had no problems with maintaining the population and indeed, increasing their population markedly without importing non-Japanese to dilute their culture over the last 10,000 years.
The Japanese are over-the-hump, with peak population reached in 2010. Now on the decline.. This means they are now regressing as a society.
Since the death of Japan first became a front-page news article back in the early 2000's, birth rates have increased, albeit slightly.... one-fifth of one percent.
That rate is insufficient to produce a sustainable child-bearing population by the time the current population leaves child-bearing age. so unless their is a major change in the next 30 years, the Japanese are in serious trouble.