Maybe the idea of rape between women not being a thing derives in part from the general conception that "skinship" between people of the same sex (Specially females) tends to include some kind of rapey behaviour by default. Played as a joke of course.
My perspective might be warped tho, as I don't live in Japan and Japanese media is not the most accurate representation of its people. But the idea repeats over most countries with some kind of western influence as far as I can tell.
Also, criminally speaking rape is the charge originated from carnal abuse... Which by definition states that the agressor has to be male.
Still, that stuff exists. Sensei, be careful with what you say.