"About 1 in 4 Japanese adults in their 20s and 30s is a virgin, study says"

That might explain the birthrate.

From Forbes:

A Quarter Of Japanese Adults Under 40 Are Virgins, And The Number Is Increasing

Having sex may have jumped the shark, at least for younger adults in Japan. A new, detailed analysis of national fertility data finds that the proportion of adults in the country who have never had vaginal heterosexual intercourse is on the rise.

For women between the ages of 18 and 39, a total of 24.6 percent were virgins in 2015, up from 21.7 percent in 1992. Among men of the same age bracket, the percentage without sexual experience increased from 20 percent to 25.8 percent.

The trend towards less sex was consistent even for the older half of the cohort. 6.2 percent of women and 8.8 percent of men aged 30 to 34 reported no heterosexual experience in a 1987 survey; those figures jumped to 11.9 percent and 12.7 percent, respectively, in 2015. Between the ages of 35 and 39, 4 percent of women and 5.5 percent of men reported no heterosexual experience in 1992; in 2015 the figures increased to 8.9 percent and 9.5 percent....MORE

Last year Japan's birthrate was 1.43, significantly below the 2.07 replacement rate resulting in an excess of deaths over births of 448,000..
On the other hand Japan's Total Fertility Rate has been inching up for over a decade.
Germany and Italy are giving Japan a run for its money among the larger states with Germany having taken the anti-fecundity crown a few years ago before turning up.

For what it's worth Vatican City has the lowest birthrate in the world. And the highest crime rate per capita.