It might have seemed like a senseless comment to you but was valid.
As Instructors if we are teaching then we are PIC and take on the responsibility for the safe conduct of the flight regardless of the perceived level of the student. As such we carry the can when it goes wrong. You did not adequately prepare before the flight, you were complacent during the flight and as a result you were punished after the flight. Those hours you get to log don't come for free, they have to be worked for. You did not work for them and made a rod for your back.
You have learnt a harsh lesson and you do have my sympathy, being a new Instructor and probably low hour pilot yourself by the sound of it, it was easy to be lulled into a false sense of security and make the mistake.
However you can now move on and learn from the mistake. Do proper pre-flight planning, including weather and NOTAM for yourself and cross check it against the student. Get a moving map GPS and learn to use it properly. I take a 496 with me every time I fly out of the local area with a student. Expand your own experience and visit more places and become familiar with them.