Movie in short:
WOMAN CHEATS BECAUSE WIFE CAN'T GET PREGNANT WITH HIM. WIFE OF A SPY AND THEIR MARITAL PROBLEMS.
PERSONAL TAKE:
I really don't understand why berating ur husband, badmouthing ur personal deficiencies of ur husband and being an adulterous woman is fine in Korean movies. Women always get a free pass for these things, but men if they do the same things r seen as horrible ppl also once they do that women r allowed to leave them but it's not the same for men.
Also, I would never trust that child is his who knows what she did on her previous trips as a air hostess or somewhere near her residence?
The probability of her having cheated is high.
I would divorce her and also get evidence of her with the dude and apply cheating, fraud and other charges even take emotional and psychological damage compensation.
I PERSONALLY WOULD NEVER CONTINUE WITH THAT WOMAN WETHER SHE REALLY DIF IT WITH SOMEONE OR NOT.. NOT TRUSTWORTHY.