So for those who don’t know in the final chapter of the manga she confesses to have used birth control. Which in turn got him to think that there was something wrong with him apart from the obvious. The doctor telling him your sex drive is in question and there family’s wanting grandchildren finally got him to the turning point that he can no longer satisfy her himself. Thus the ntr but this in turn caused her to changed as well. The events that followed changed them both and continued their ntr life.