The article says that the sexual slavery originates from Yoshiwara brothel built by the shabby licensed prostitution of Japan.

It goes on:

Japan has a 760-odd-year history of licensed prostitution from 1193 to 1958.

Japan's licensed prostitution was the perverted one quite different from a shady bar in European and East Asian countries.

In conclusion, the sexual slavery for the Imperial Japanese Army was the military feudal sexual slavery that forced women in areas invaded and occupied by Japan, mainly Korean women, to offer the sex. The system was based on the state-regulated prostitution of Japan with a despicable historic tradition and the idea of suppressing women, the extreme national chauvinism.

In other words, the "comfort women" system for the Imperial Japanese Army, modern-day sexual slavery system, was established on the basis of the despicable traditional sexual slavery system of Japan.

Such slavery was unethical one beyond all descriptions as the sexual slaves were punished, tortured and killed so often owing to contempt for residents in colonies, exclusive feelings and military and feudal cruelty.