Han Chunhee wasn't prepared. He knew deep inside, it was the plot acting up again and usually, when a grand thing suddenly occurred, it meant two possible things: the protagonist's desires were becoming extreme so he was doing his best to progress at a speedy pace, OR the protagonist's opponent or enemy was up to something to stir trouble again for the protagonist.
When he faced Yujin that day and heard Yujin's proposal out of the blue, Chunhee's first reaction was utmost disbelief, followed by an immense feeling of mixed delight and worry. He knew he was happy but at the same time, he was greatly worried. The feeling made him unable to celebrate or mourn. It was definitely a chaotic feeling.
Yujin was dead serious.
He wouldn't even take a 'no' from Chunhee. Or perhaps, he was but at that time, Chunhee himself was unable to do it.
Chunhee agreed. And the rest of the population discovered about it immediately after.