Obviously he broke up with his girlfriend already. It doesn't have to be stated since it's already been implied a lot of times.
I also think this story started as a PWP but the second seems to be developing something and I'm looking forward to it.
= couples who are blood-related or know each other somehow otherwise
additional are the series in the following lists:
http://www.mangago.com/home/mangalist/6968/
http://www.mangago.com/home/mangalist/7543/
http://www.mangago.me/home/mangalist/8347/?filter=&page=2 (just the Kuroneko series)