Falsely accusing another person of a crime such as rape is a serious crime. Because rape is a serious crime to begin with.
Because rape is such a serious crime, courts tend to look for at least some outward/objective evidence that intercourse was indeed forced. (Tbh, if rape was not such a serious crime, more people would be found guilty. That’s the reality with all crimes.)
Under Korean criminal law, the max punishment for false accusation is 10 years or 15 mil won. In practice, guilty people are normally sentenced to 6-8 months. More than 2 years is rare. (Criminal fines are also very common.)
FYI: According to prosecution figures, there were 5,386 instances (in 2015) of people being found guilty of false accusation (in Korea). About 14.76 per day. Of course, this figure is the total number of all types of false accusations, not just people falsely accusing another of rape.
Finally, it should be noted that rape and false accusation are separately-reviewed crimes. Just because a person was acquitted of rape does not automatically mean the accuser ought to be found guilty of false accusation. If the accuser honestly (and in good faith) felt he/she was raped, that is not false accusation.
Thanks for reading!