Honestly, the humour is very intelligently set up.
Author shows main couple being intimate in the council room
Uses someone who already knows they like each other and wouldn't interfere to 'close' the room
The pesticide sign seems a little contrived but is ultimately rewarding as it leads into another small joke:
Ishigami and Iino come to eat together in the room to avoid attention, Kashiwagi decides the best way to get them not to come in is the sign, but instead mentions that a Buttersly is emerging.
The pay off is ishigami shouting that with all the pesticides, it would be in danger.