The controversy reveals a South Korean public far less wedded to the idea of inter-Korean unity than previous generations, analysts say, a changing dynamic that may shape South Korean President Moon Jae-in's efforts at reconciliation with the isolated North.
North Korea's participation in the Olympics has been seen as a win for Moon, who hopes to use the event to make a diplomatic breakthrough in the standoff over North Korea's nuclear and missile programme. It also eases public concerns the North might upstage the Games with yet another weapons test.
But Moon's specific moves to integrate the two Koreas at the Pyeongchang Winter Olympics have sparked a sharp backlash that goes beyond his traditional conservative detractors to include his main support base of younger South Koreans upset an unchastened North Korea is stealing the spotlight.
"North Korea was all about firing missiles last year, but suddenly they want to come to the South for the Olympics? Who gets to decide that?," Kim Joo-hee, a 24-year-old translator told Reuters during a coffee break on a chilly Seoul afternoon. "Does North Korea have so much privilege to do whatever they want?"
Moon's office declined to comment beyond saying the two countries would be coordinating logistics for the Olympics, which begin on February 9.
Opinion polls released since the plans became public have shown limited support for some of Seoul's proposals.
Only four out of 10 respondents said they favour the plan to march together under a flag symbolising a unified Korea, according to a survey released on Thursday by the South Korean pollster Realmeter.
Tens of thousands of people took to social media to vent their disgust after plans for the joint activities were announced on Wednesday, with one commenter saying the Korean peninsula flag is "not my goddamned flag".
Others complained "the Pyeongchang Olympics have already become the Pyongyang Olympics".
