"whiskey" writes in a comment:
Japanese women are not having sex because they'd prefer more macho, and socially dominant men. Few of them exist in Japan, because modern industrial nations depend on male cooperation not "Big Men" that women desire.
Completely wrong if you believe this article:
Both Shirakawa and his girlfriend like the fact that she weighs more than he does, and is the leader of the couple. "She's a lot stronger than I am, can lift heavy things and go drinking until dawn. I admire that about her, and feel protected when I'm around her," he said. Older than he by five years, it was Shirakawa's girlfriend who made the approach, started the dating process and decided what course their relationship would take.
Supposedly, the ideal size for a young man in Japan is 5'8" and 125 pounds. I'm not sure if I buy into that.