Where I live, no-one has (or would even consider having) net curtains, therefore, unless you have shutters or blinds, you are open to the world.
My postie and courier services know by now that I may take a short while to open the door if I'm in my study on the 2nd floor. I do call out when I'm on the stairs and they are good enough to wait. As my front door and that of my neighbour can only be accessed via a 'right of access' gravelled drive, we don't suffer from random callers...thankfully. I always think it's nice in passing to glance into sitting-rooms which, on winter evenings, look welcoming and cosy but, to have intrusive behaviour such as the OP and others have mentioned, would be a step too far and right out of order.