As promised, further info on Japanese rape laws:

Sexual Harassment and Domestic Violence in Japan - Yukiko Tsunoda

Unlike the U.S. or western countries, Japan has maintained an old-fashioned rape law under which a victim is still required to show evidence of having resisted. Also, a victim's tacit consent or the rapist's misunderstanding of consent is still accepted as an effective defense by the legal system.

This is not the original article I read, which was attached to a Time.com article on the original reporting of the Okinawa rape case (and is no longer available at their web site without paying a fee). However, it provides the same information, and a whole lot more. The author is a Japanese attorney. The article itself is date 1997, so it is possible that some things have changed.