
Omg now my rating goes down, i hate that 2nd story. I hate shuuya, he can easily change partner.
Like, he isn't serious with kouhei (╬ ̄皿 ̄)凸

Don't be silly. He's just hurt. He didn't change partners. He is serious with Kouhei but thinks Kouhei simply sees him as a cheap sexual thing

Were you even reading the same chapter as everyone else?
You totally missed the point.
That was the first time Shuuya took the courage to take the initiative to please his loved one and try the riding position. He wanted to make his partner happy, of course he is offended and hurt that his boyfriend is only thinking about how he might have done the same in a previous relationship(which he didn't). What Kouhei did by saying that to Shuuya was just treat him as a sexual object, Shuuya has every right to be upset at Kouhei.
How did he change partners?
That was a flashback of a previous relationship where he just thought about making his partner happy but it just turned out he was being used.
He just slept over at Kouhei's father's house, nothing sexual happened.
How did you get the idea that he had changed partners? Are we reading the same thing?

After re read this i just realize something is off
http://www.mangago.zone/read-manga/samishigariya_no_love_letter/bt/343168/Vol1_Ch4/21/
Not enough depth of the feeling
But after all its good