Monty isn't giving you new information - he's giving you an extra "pick".
If you never change your choice, you can completely ignore anything Monty does, because your first choice is always going to be 1/3 that you picked the right door.
You only have 2 options, but that doesn't mean your odds are 1/2.
Because Monty opened door 3 for you, if you change your choice, that means you have a "pick power" of 2 doors, because you get to choose both doors (2 and 3).
Basically, your two choices are:
Door 1 - odds = 1/3
Doors 2 and 3 together - odds = 2/3