He could've at least closed the door with lock on it or maybe be a little more quiet about it since he has have a barely adult at his home. （￣へ￣）
Do we even know that he left the door open? But also that house is huge and it's nighttime and Sejin should be sleeping not going to someone else's room
We know he has some sort of anxiety, and he wouldn't be able to sleep, and like I said, he should've put lock on it. Maybe Seijin thought of something like "I wonder how they're doing" or "I hope he did what he told me about sleeping on the living room" and he went to check on them but didn't expect to find that. Maybe he couldn't also sleep because of the noise?
Then that's on him. Adults are allowed to have consensual sex with some expectations of privacy in their own home even if they live with others. Let's all remember that Sejin put his cum stained underwear in the communal laundry hamper - an actual violation of boundaries
Why is everyone acting like he's committing a crime by having sex in his own home? People do this all the time it's literally normal. I get that something bad's expected to happen next chapter but at this point he's done nothing wrong