I'm going to make a few assumptions here, but my thought is that women in porn largely look like they're between the ages of 18 and 30. Men on the other hand look like they're 30-45 even when they're 'acting' like they're a lot younger.
Guys in porn are usually quiet and the focus is on the woman because we're supposed to be able to imagine ourselves as the guy. Kind of like the master chief. You aren't supposed to see his face because you are him.

These Japanese guys want bodies that resemble theirs more, so they can think of themselves in those positions easier.