That Kunoichi does duty by sexual charm seems to be a fiction.
Kunoichi are sent to the castle or mansion and work as maidservant. Kunoichi will collect various information and report it to allies. Notions dealer and kimono dealer go to the castle or mansion and they met this woman ninja and received inside information.Since Kunoichi returned to its home periodically, information may be then given. Their family and a guarantor are fellow ninjas and the family of Kunoichi was trusted by the surrounding residents so they can easily do intelligence.
The beautiful girl Ninja is sent into the castle as a mistress where a enemy's feudal lord lives.The girl Ninja who is in the inside of a castle has clothes and household goods sent from its home. Then the large clothes box made of wood will be sent from home.The bottom becomes double, and when the inside is opened, usual clothes are included, but male Ninja hide himself in the lower bottom.The ninja was able to steal into a castle safely in this way.This method is called "Kakuremino no jyutsu".
Japanese Castle
There were a lot of children who lost a parent by war in Sengoku period.Samurai lord made those children Ninja.The girls become Kunoichi and travel everywhere as shrine maiden.Without being suspected as Ninja, the shrine maidens were able to travel in the whole country and was able to gather information.
shrine maiden