I think the ML was already bewitched/in love with the FML. That's why she couldn't erase his memories or because he's her fated partner. Even in chapter 1 when the FML asked whether she was ugly, the ML hesitated before answering. Regarding the ML sleeping with someone else, I guess it's because he as her childhood friend witnessed her constantly with other men while treating him just as a friend not as a man. Maybe he slept with others to get over his feelings, to maintain their childhood friendship.
I think the ML was already bewitched/in love with the FML. That's why she couldn't erase his memories or because he's her fated partner. Even in chapter 1 when the FML asked whether she was ugly, the ML hesitated before answering.
Regarding the ML sleeping with someone else, I guess it's because he as her childhood friend witnessed her constantly with other men while treating him just as a friend not as a man.
Maybe he slept with others to get over his feelings, to maintain their childhood friendship.