Most countries are underdeveloped countries that do not possess such widespread forms of media, and the people are often too busy surviving to care about morals. These countries are also the same countries where children work and the age for marriage is puberty. Before you spout crap like that, perhaps you should know what most of the world is, because it certainly isn't all developed nations like you seem to believe it is.

The concept of child itself is a relatively new concept when compared to human history. It was developed during the industrial revolution in which literature began to define children as a set audience. Before that, children were simply considered little or untrained adults. They worked like the parents, learning how to do their roles, and they wore the same clothes (just smaller). There was no such thing as a child before the industrial revolution, no such thing as children's clothing, and fairytale/folklore were designed to entertain both adults and children alike (which is why many of them are darkly themed and often contain lewd innuendo). It is only recent that fairytales became censored for children and made more "PG", but in fact, in most of these fairytales, princesses getting married off, kidnapped, or whatever would be 14-16 years old, sometimes even younger. (Side note, romans didn't even name their children until they reached a certain age, because the odds of them surviving to adulthood was small). So in case you can't make the link here, children have been put in explicit situations long before the concept of children even existed, and these were passed down through fairytales and folklore (heck incest seems to be a common theme as well).

As for the content of children in explicit situations, the banning of such can serve to make the act more widespread in modern society rather than restricting it. Similar to smoking and alcohol that is "the cool thing" to do amongst teens, marijuana that "should be legalized man", or even prostitution, making something illegal only serves to glorify the act and subject it to misuse. The misuse of alcohol even in Canada is significantly different in anglophone provinces compared to quebec, becuase the traditional francophones allow their children to have a glass of wine with dinner. When they are freely taught and given alcohol, they were less likely to binge drink or abuse it, whereas for the rest of canada, there is a high amount of underage drinking and binge drinking. The act of making alcohol illegal has glorified it into something "only adults can do", so teens that believe they are more mature will try to drink to impress the people around them. With such limitations on access to alcohol, they have no choice but to drink a lot because they don't know when the next time they may have some, and after experiencing the "joys" of being drunk, many of them will try to drink to get drunk more often.

Marijuana, people are trying to legalize...it's a vastly covered subject that doesn't need to be covered again. Prostitution is illegal, yet the system of law has somehow become a crooked system in which prostitutes are victimized rather than the pimps! Most prostitution occurs out of necessity. There aren't many women willing to sell their body unless they are forced too, or unless they must in order to feed themselves and or their children. But by making it illegal, the system has instead made them unable to go to police when they are being forced or mistreated, and unable to seek protection. If prostitution was legalized, it could be restricted to specific regions of the city, customers and employees could be safely screened for STD's, and the employees could be protected under the job's act for health care and fair wage! Instead, because "human moral" finds anything pertaining to sex to be immoral and wrong, prostitutes are shunned and victimized, because after all, the abuse of women is okay, as long as they are immoral, right? That must be the case since that's what social morals is spelling out.

Do you think banning illicit content in mangas is going to stop those that like it from getting access to it? Pornography in japan is supposed to be for adults but teens manage to get it (in the west too) and in fact pornography is censored in japan, which makes those that want nudity to either look for uncensored pics, or satisfy their sexual lust in other ways. Because of the strict censorship on pornography, there is a greater incidence in sexual assault (to the point where some trains are gender segregated). In a study that I had read in the past, it was found that pornography actually served to DECREASE the incidence of sexual assault and rape, because those that would otherwise commit the act had access to material that could satiate their lust. While the thought of creepy fat people sitting in their room masturbating may be off-putting, IT IS A LOT BETTER THAN HAVING THEM ON THE STREETS ASSAULTING PEOPLE. In the end, the study found no conclusive evidence that pornography would increase the incidence of sexual assault, or that those that watched a lot of pornography were more likely to become sexual offenders. Instead, sexual offenders often originate from those that had their sexuality oppressed in some way (family, society, or religion).

Tying everything together in one neat little bow-tie:
Children depicted in explicit ways has always occurred in history
The banning of things often serve to glorify it and subject it to misuse
Public morals on sexuality only victimizes the vulnerable
Pornographic situations serve to calm the beast within

And the illicit content bill in Japan is a MOOT bill simply made to make the government more moralistically wholesome while giving them more power to censor and ban things.