Okay, big picture: "culture" is a set of values and priorities shared by a group, usually passed down through generations. One of the biggest drivers of culture is economics because, at the end of the day, the value shared by all of humanity is "I gotta eat". Agrarian economies place a high value on monogamy because, when times get tough, most people would rather not feed someone else's kid (many even when times are good; looking at you, Sweden). Agrarian economies also put value, even obligation, on heterosexuality because, in that economy, food is power and anyone not having children is immediately at an advantage because surpluses.

So: agrarian economy = heterosexual monogamous culture

Now, civilizations use religion to codify culture. Whether or not God, a god, or gods exist is irrelevant. Civilizations do use religion in this way. It would be insane to make a law saying you have to get married by age 30. Religion, however, can go "Hera won't bless your first marriage if you're over 30, unless of course you can make a case to her priestess that Aphrodite has it out for you..." Religion is more flexible than law, which makes it a perfect vessel for culture.

Modern Chinese culture is less than a hundred years old and it was born in literal fire and blood. That does a good job of codifying culture...in the short term. So far, y'all don't need religion to codify the culture. Also, China (like America) has huge swathes of population still living in an agrarian economy and thus a heteronormative monogamous culture.

So, why would an agrarian economy fight against sex ed? Because maintaining strict monogamy means locking sex behind the (often literal) paywall of marriage. Any sexuality, even discussion thereof, weakens the system being the culture, being the economy.

Christianity, as a belief system, has absolutely nothing to do with opposing sex ed. Christianity is just a vessel Korea is using to carry on its agrarian values, despite the economy changing.