@Noel_S_Leitmotiv I thought you were gay. Peculiar.
Anyway, as many have said, sane, morality, ethics, creed; all of those are relative to cultural environment. Of course we find is unusual.
We’d do well to remember that Japan is a very “locked” country in terms of outside influence, tradition and culture. Apartments are quite often very small and packed next to and on top of one another. Without getting too graphic, you have to be very quiet whilst having sex and, if what I hear is correct, it’s quite uncomfortable because of that. Combine this with Otaku culture and, well, of course people will look towards other objects to fulfill their emotional desire. Arm-chair psychoanalysis aside, I’m under the assumption that this is just an extension of that;