http://www.mangago.zone/read-manga/kobi_no_kyoujin/mf/mi_chapter-13.7/pg-36/ Err, i dn't ...

Maybe she is still able to have a child?

I thought it was some kind of way to pressure his father, like: "you want me to do something I don't want, so you should do it too".
Because his dad's wife was his previous mistress, so even if it's expected that he has mistresses, he shouldn't divorce(?).