The link to the article is right here. It’s entirely in Japanese, but there are some graphs you may be able to understand with a little Japanese ability.
To put it straight, the article is not a very good piece of scholarship, the first graph the article provides (on the first page of the link above) is a chart of abortion rates. They argue that since the young women who’ve had abortions probably had partners in the same age range, this data is applicable to young men as well. (Hopefully, faithful reader, you see the flaw in this logic.)
The steady drop in abortions in Japan between 2001 and 2005 is offered as evidence for the decrease in sexual desire in young men in Japan.
Other evidence offered are the rates at which people claim to have “sexless” marriages, as well as the opinion of a freelance writer who is an expert on the sex trade in Japan who says claims that a decrease in the number of 20 somethings is also a sign of the decreasing sex drive of Japan’s younger generation.
I hope it just means that they are staying home more, having less abortions, and being more discreet about their private lives, but who knows?