The sheer amount of Japanese porn on the net is astounding, already. If you look further into it (For purely scientific reasons, of course), then you quickly will realize that the Japanese are either the most kinky society on earth, or the most tolerant and opened minded...it might be both.
The Dutch might have them beat when it comes to "live action" though.
I think generally Japan/"the far east" in general has a number of things going on. The BIG one is it doesn't have the direct cultural imprinting of the Judeo-Christian faith stretching back centuries into the past, at least as a population as whole. (Enclaves may be another matter)
Japan and S. Korea have a weird double whammy in that they went from "traditional society" to being brought forcibly into a western mold at the close of WW2. Of which Japan received the brunt of it under McArthur where a lot of "Christian ideas" were imposed upon them externally. (The most notable casualty for the "alternative lifestyle crowd" on that front was the change that happened regarding hot springs and mixed bathing. Mixed gender, nude bathing was commonplace, right up until the end of WW2 and the occupation of Japan by US Troops--with US (christian) cultural mores and expectations.
Watching Anime/Hentai, or their print counterparts, you'd be led to believe gender segregated hot springs/bathing was the "time honored tradition" that went back centuries, rather than the decades it actually does.
BUT concurrent with all that, the 1950's also saw the rise of Playboy Magazine, and the numerous other porn rags that followed it. The "sexual revolution" of the 1960's and 70's had to be all kinds of confusing from the Japanese perspective as they tried to further "westernize" their society. In that context, it's no small wonder that Japanese society seems to be all kinds of screwy, even by American standards, when it comes to sex and sexuality.
Making things even more loopy on the Japanese side, which makes NTR kind of make sense, is their comparatively recent departure from being a feudal society. In particular one with a long and rich history they have every right to be proud of. (As well as now having the ability to be fully aware of the seedier bits as well with comparative ease.)
It is that feudal link which makes NTR seem a bit less extreme in their context. Dom/Sub is simply their transposing the Lord/Liege relationship, and its off to the races from there.
Throw in modern feminism, something else that they likely "imported" from 1950's/60's America, and you have the classic setup for "disempowered male" who "is worthless" and "completely at the mercy of others"(even women) which basically sums up NTR from the male perspective.
So basically, it's a bunch of sexually frustrated Japanese men QQ'ing over the "decline of the patriarchy" back when "men could be men" and why the focus is on "you're not man enough to _____."