I don't remember what this yaoi manga chapter is about. I can only say details. Okay so the protagonist in the beginning of the chapter walks in on his brother (cousin?) in his apartment with another guy (after they have sex). The brother (cousin?) and his boyfriend are under a futon, half naked, getting dressed and there was tissue everywhere around them. The protagonist who walks in on them is in total shock. And also in the manga the boyfriend of the brother (cousin?) is thug-looking and the protagonist doesn't like him.

If anyone knows what manga I'm talking about, I'd really appreciate it if you told me.

Hm, I remember something like that, but I need more details to actually remember the manga's name.
You could try the Caught In the Act too...

Doesn't fit your description perfectly but I'm gonna try anyways. Sawattemo Ii kana Probably not it though 🙁

Sorry but none of these is the manga I'm looking for. Can anyone keep looking please? Much obliged.

"A Single Futon" by Kunieda Saika?

Thank you all so much.