Yeah, it is just cheating by her own wil because she felt horny. She isn't being stolen away or something like that.
Here is summary; The fat guy had a crush with the main heroine in the past but got rejected, years later they meet and he, a teacher, is dating her daughter no less. Her daughter helps the teacher to subdue her mother and make here watch as they have sex. Since obviously her sex life is not satisfying she starts liking it. So like that she starts her new sex life having sex with various men including the rape train, the school staff, guys in a park, a shota and his father. Edit: Last chapter(Mugen tensei 15-11) Time skip, heroine's daughter is pregnant and has sex with the teacher while she watches, she hasn't been having sex with the teacher since then or any men.
Heroine is sexually frustrated, and tries to make her husband have sex with her but he isn't willing. Then she tries getting him drunk to make sex easy but he falls sleep hard.
Unsatisfied starts masturbating, then the teacher appears and teases her, revealing he always knew since school she always was a pervert and exhibitionist. She can't hold on and accepts that she is M as they start fucking. Time skip, she is pregnant like her daughter, both from the teacher and have sex. After delivering their children, the heroine has sex with various men while the husband is away. End.
Also there is no lactation not even in the parts with pregnancy.