What?

His ex blackmailed, manipulated and assaulted him. It was all in the flashbacks in chapter 4. He met Misaki in High School. Chiaki had posed nude for a shady guy to earn money to help his dad, but the guy posted the pics online and Misaki saw them. He used the pics as blackmail material and said he would tell Chiaki's dad about them if he disobeyed him. That was the basis of their relationship. Chiaki is despondent over his situation and realizes he is in love with the fat version of Kou. This is where Chiaki gets his attraction to fat men, they remind him of Kou in High School. Misaki first assaults Chiaki after he notices his friendship with Kou and gets jealous. When Chiaki's dad dies, Chiaki can now be free of Misaki and he breaks up with him. He tries to have sex with other men who remind him of Kou, but he realizes it isn't what they look like that matters, they are not Kou. Shortly thereafter, he meets up with the new, thin version of Kou while waiting for someone from a dating site. Chiaki thinks Kou is the person from the site and they have sex and go on a date to a wrestling match. Chiaki figures out that this person with him is Kou, probably from his speech and mannerisms. They have both been in love with each other since high school, so things go somewhat smoothly until Kou finds Chiaki under a fat customer. They fight and make up when Chiaki dresses as a girl. At that point they become an official couple.

No sorry maybe i should of been more clear. Will kou ever find out what happened? And again why show the scene of the ex cutting his hair if its never going to lead to anything?

There is a spin off. Hope we'll have it some day.

Kemono Asobi

Is that a part two

It's more like a spin off but this couple makes the appearance in recent chapters

Thanks