Sorry I don't remember anything about the actual main story of the manga.
However, I remember the extra chapter.
The extra was about the seme (dark hair, black?) Imagining what it would be like to have children. He thinks of having a son, but his imagination shows the son (who is just like the father, just smaller) going after the mother (the uke) forcing the uke saying something like, "Father is not here" and the uke calling the seme for help.
Then the seme starts thinking about having a daughter who looks just like the uke but smaller (with uke's light hair and all) and the seme likes the idea.

Sorry, I don't remember much. I just remember the extra.