The Japanese just have a lot of weird ideas when it comes to sex. I say "weird" - they're weird to us, I'm sure it's perfectly normal to them. Now some of this is learned through osmosis through lots of the porn stuff, so I'm sure some of it has to be taken with a grain of salt. But there are a few other odd things too, that I'm certain are something that actually happen at some reasonable rate.
I remember reading an AMA on Reddit, and the celebs in question were three Japanese porn stars - I forget the names. One of the questions was from an American who had lived in Japan for a while. One time, he was having sex with a Japanese woman, and while it was going on, she said "stop". To any westerner, this is an obvious sign to stop, see if something's wrong, did you hurt them, etc. And that's exactly what he did. But she really didn't want him to stop. But she was almost acting like it. He was asking if this was a common thing in Japan.
Turns out it's relatively common. I forget the exact answer now, and I'm actually super-interested, so I want to go back and look for it later, but it's kind of a strange thing.
Thanks for the explanation, the idea is odd at best.