RBD-704 Rina Ishihara's secret to keeping her lover

Rina Ishihara is a young married nurse, but she has caught the eye of a man who has a hobby of possessing the person he likes and only for him. The secret to helping him do that is a chastity belt with a private lock that only he knew about. Why was she bound by that sacred belt every time she had a secret affair to the point that her husband had to give up?