@Jay Another possible interpretation (and the one I think makes more sense) is that Haegu never had those morally questionable dreams to begin with. Maybe she only recalls, hazily, the fragments of what happened that day with Mian. We see in the flashback that she was almost in state of semi-consciousness (when she wasn't completely unconscious). She was in great pain and she couldn't think clearly, even wondering, when she first saw Mian in her room, if that was a dream — she couldn't distinguish, at that time, what was real and what wasn't. Mian did all that stuff while Haegu was in this state, and Haegu could only watch and wonder if that was really happening or not. But remember she saw everything: Mian kissing her, laying with her on her bed and touching herself. Mian later manipulates her (in the present), saying Haegu had been dreaming of her sexually for a long time, when in reality she's just constantly recalling that situation, confused as to whether that was something that really happened in the first place or not.