Responses
I don't think it was Shirashi having sex in the room next to them. Why would he say beforehand "if it continues tell me right away" and then get caught by Takashi and Minase again rightaway. Also on page 8 of chapter 2 it looks like Shirashi scolded some other students of using the room for sex.
At least that's how I got it. But the plot seems to be a bit messy tho.

This is what i understood from chapter 1: Minase (MC), fell in love with his younger brother(Tetsuo)'s friend Takashi, but had to move away. Flash forward to current time. -texts between Tetsuo and Minase about Takashi arriving soon- Shirahashi (dorm manager and friend of Minase) is introduced shortly before a new dormmate, Fujiwara Yukifumi, is introduced as well. There seems to be emphasis on the fact that Yukifumi sounds like Tetsuo and Minase thinks he is still in love with Takashi. Takashi arrives shortly after and the dormmates are split with Shirahashi and Yukifumi in one room and Minase and Takashi in another. -texts between Minase and Tetsuo with Tetsuo telling Minase he's too busy to come visit- Minase and Takashi are going to bed when they hear noises from the room next door, they get as far as the door to the room before deciding to go back to their room when they look in the room, but see no one there. There is a smell though. The next morning the four of them meet up to eat and discuss Minase's sweets-making hobby, it is revealed that he does it because "the person he likes wanted homemade sweets" which actually ends up being a misunderstanding because Takashi actually doesn't like sweets. It is revealed that Minase has a case of amnesia or maybe short term memory loss, he had thought Takashi liked sweets because he came over holding sweets back when they were kids, but he'd actually forgotten it was Takashi's mom who sent them over. Minase tells Shirahashi about the noises he and Takashi had heard last night to which Shirahashi replies saying "if you hear it again tell me right away". Minase and Takashi are going to bed when they hear the noises again, Takashi suggests that maybe it is someone having sex as he'd heard rumors of a "sex room" around there. Minase and Takashi resolve to go confirm that it's sex so as not to false alarm Shirahashi and see 2 men having sex in there(honestly, it looks pretty obviously like Shirahashi and Yukifumi). They go back to their room and resolve to tell Shirahashi tomorrow when Takashi suddenly kisses Minase before heading to sleep. -End of Ch 1-