it's pretty hard to mix up china and the church. but you are mixing up one child policy and sex taboo.
chinese buddhism and confusianism considered sexual lust a, well for lack of a better word, sin. taoism codes also calls for quieting one's lusts. folklore believes that semen is the essence of man, the cream of blood. so sex, the process in which semen was expelled from the body, is a process of losing one's vitality. it should be done out of necessity. this was the traditional chinese mindset on sex before the communist tookover.
after the tookover, the reason for porn ban was written in the party code way before there's internet porn or one child policy. in the early days of the PRC, proletariat/revolutionary morals were enforced. bourgeois luxury such as wine and fine dining and good clothing and, well, sexuality were discouraged. in fact trend for women in that era was to dress as much like men as possible. if you look at the chinese bank note--i believe it's a 10-yuan note--in that era, the woman on it was driving a tractor (to signify that women are as good workers as men), and pretty much dressed like a man. the chinese were told that sex is a means to boost the army of revolutionaries. and woman with many kids were celebrated as hero mothers.
then after the cultural revolution and death of mao the deng govt realized china had a population problem, and one child policy was established. this was in the late 70's. it's a policy of necessity. early enforcements in rural areas were quite brutal because there's fierce resistance from the son-loving peasants. toward the one child goal, contraceptives were given for free or sold publicly, and when it's sold, it's called "equipment for planned reproduction".
one child policy or hero mothers, the moral smear on sex is firmly in place, written in law in the form of ban on porn and prostitution. it's ironic, because though illegal, prostitution is rampant in china right now, and often corrupt officials have a stake in it by turning a blind eye. example, wen qiang case in sichuan. that guy's lover actually ran the biggest prison-brothel establishment in the city of chongqing. google it up.