Responses
I believe you are looking for chapter 6 of the manga Yachin Hanbun No Ibasho Desu
http://www.mangago.zone/read-manga/yachin_hanbun_no_ibasho_desu/mf/v01/c006/1/
This is it! I'm really thankful to you!
I read this manga. I don't remember much but it was about cheating seme (he was doing it on purpose). Uke was forgiving but one day he saw nails' marks on the seme's back and got mad. I'm not sure but they had a friend that uke went to when he got mad at seme. I have been looking for it for a long time. Please help!
PS. Sorry for my English :)