I think the culture is way to complex to find any one reason. Things may have changed since I was in Japan, 15 years ago but I can tell you what I have noticed.
There are several different things going on.
1.They find it fashion forward to adopt European ideas and styles. Porn and the sex trade is very big in some European countries.
2. Japan use to have Geisha girls for many years before porn. It was an accepted practice that rich hard working men, had Geisha girls for sex and the wife for heirs.
3. Although Japan has educated their women, there is still a strong bias and resentment towards working women. The old boys club is still very strong. That could be some of the reason for the violent porn they have.
4. If it makes money and sells. Then they are going for it. Japanese porn isn’t only made for the Japanese. If anything, their men find European Blonds exotic and Japanese females dull, while there is great interests from Europeans and Americans, over Japanese women. And the same can be said for the women. They find other cultures fascinating.
5. From what I understand. Their is also a new culture being born, where Japanese men and women do not want to marry or engage in sex. They are working hard to simply survive and find that relationships get in the way of, the little time they have, to have fun. Housing is over crowded, the work force is overcrowded, and property is expensive. They are either prolonging getting married and having children, or just not interested in ever getting married. My guess it that, it will make for a lot of sexually frustrated guys and porn is the short term answer.