Bans like these make moralists feel good, but rarely work as planned. The evidence of porn being the culprit of the widespread of "choking" is weak. Once again, correlation and causation have been mixed.

Restrictions on free speech should be rare, narrow and well defined. This is neither. It is both broad, and unhelpfully vague. The government has not clearly defined the acts that are illegal. Instead, it expects the courts to decide on a case-by-case basis. Adding additional burden to an already overloaded legal system that has better things to do than to define if that use of "mommy" or "daddy" in a porn video constitutes incest.

Since possesion of banned content can be punishable by prison, it also dilutes what is means to be a sex offender. Such important label loses value when it is applies both to serious sexual offences and someone with a stepmom fantasy.

What the government should be focusing instead is on better sexual education. They should focus in teaching about safety, consent, and how to talk frankly to prospect partners about what the other person does and doesn't want. Sex education is dismal in most of the world, in Britain it is better but there is still room for improvement. If young people do not learn about sex from reliable sources, they will do so from unreliable ones.

Pornography is a difficult thing to defend, particularly in public. But Britain has become too willing to ban anything pearl-clutchers dislike instead of adressing the root cause of the problems. A ban on kinky porn will please moralists, but it should alarm anyone that is serious about free speech, or government overreach.

Looking at the Wikipedia article on Pornography in Japan, the most recent censorship on pornography in Japan has been in effect since after the Allied occupation of Japan.

The article also seems to indicate that most of the ideas regarding censorship were introduced by Westerners to the Japanese society during this period:

The Japanese public slowly came to adopt some of these ideas and practices. Negative views about pornography that were foreign to Japanese culture were accepted and applied to visual depictions as they were the ones most likely recognized and thereby criticized by Westerners

As for non-censored Japanese porn, I believe those are created for a foreign audience and distributed outside of Japan. Specially since the relevant article only seems to concern distribution.

Under Article 175 of the Penal Code of Japan people who sell or distribute obscene materials can be punished by fines or imprisonment.

As for why it has not been changed, my theory is that the Japanese society is very conservative and it is not one of their priorities right now, particularly with the current wave of right-wing movements across the world.