North and South Korea are in a permanent state of war; of course their border is militarized. China and North Korea, on the other hand, are historical allies. There are actually very few border controls, and vast tracts of land that one can get across relatively easily.
With the shortage of women in China, one of the options is to buy a North Korean bride. The money will be paid to her family, and she'll be smuggled across the border into China to join her new husband. Since she enters China illegally, she'll have no passport or visa, and thus no legal rights; her husband can do pretty much whatever he wants, and she has no recourse to seek help.