Kasumi cheated on you because she felt you where never paying attention to her. She tried to tell you that she want more time with you, you never gave it to her. She would cook for you all the time and clean the house but you just came home after work and went straight to bed. She thought you didn't care for her so she tried to find validation by other men but she knew that it was wrong so that's why she always came home late.. she felt guilty but didn't want to lose you