...I didn't expect much from this manga, I read it for the lewd romcom, but it make no sense that the MC enters the room of the girl he sent to change clothes without knocking to bring her a cookbook he found in the kitchen, the only place to leave one in the whole house.
He knows this girls are in heat and do that, the whole part make little sense, who would have a -busy/not busy- sign on their own door? a big house like that could place a name tag on individuals room, that made me think it was really the bathroom but the MC entered, without knocking, searching for the girl, that I want to remember he sent to change clothes, to bring her back her book and even telling out loud the reason.
As he cited to have a sister I can't believe he would just do that in a stranger house, the mangaka was sadly unable to make a good reason for that plot development, that is really sad because is a cliche old as time.