Well, firstly I think the correlation between religion and fertility is probably a lot more relevant after the invention of medicine, sanitation and contraceptives. A couple of hundred years ago a lot of people didn't survive to adulthood. Certain religious groups such as Mormons have massively increased in proportion over the last hundred years.

As for the Chinese, over short time periods memes win over genes, and Mao stamped down on religion. Does Confusianism/Taoism/Buddism have any sort of similar teaching about being fruitful? Are these even religions in the same sense, or are they legal codes, an early attempt at science and a philosophy respectively?