I am uneasy about this. I don’t see the tight coupling between watching and not unpinning. Watching implies you care A LOT about new stuff, it does not imply you are asking to see the topic first up in the list every time. That would be a “per user pinning” feature which is a can of worms I don’t want to open.
I wonder if you should just go ahead and disable the auto unpinning altogether in your forum? Isn’t that the simpler approach to take.