I want to know too, i think ead it but i forget
Its because his been taking birth control and heat inducers for to long those have side effects and caused difficulty getting pregnant or make you unable to. Its sad cause i want them to have babies (︶︿︶)＝凸
I think it is chapter three when they meet Keisuke's dad. Karasuma used and abused birth control drugs and so his body is not equipped to carry a child. It is unclear if he will be at a better state to have one or if it is permanent.
Sorry if my question not related to sumi and utou, but I'm curious why keisuke and his pair can not build a family or have kids??