Are you obliged to have the left indicator permanently switched on when driving straight ahead due to passing of parked vehicles.

No, the indicator should not be kept on continuously. According to Art. 22 para. 5 of the Road Traffic Act, you must stop signalling a manoeuvre immediately after completing it. You should switch on the left indicator before starting to pass the obstacle, and then switch it off after changing your course of travel to avoid misleading other road users.