Not true for most 6-7-8 speed chains. You should be able to reuse the pin, just make sure that the side plates are not distorted after reassembly and that the chain pivots smoothly at the pin. You may have to flex the joint slightly side to side to prevent it from binding.

All 9-speed and up chains (that I'm aware of) have swaged pins, where the outside diameter of the pin is expanded to hold the side plate in place. You can usually tell if the pins are swaged because there is a dimple in the center of the pin. These pins cannot be reused. For these chains, you must use a purpose-made replacement pin for that brand/size of chain, or a master link.

Since the OP has a 7-speed bike, I believe he should be able to break the chain (making sure the pin is not pushed completely out of the second side plate) and reassemble using the original pin. Alternatively, he can remove an outside link and replace with a master link. Personally, I always use a master link and I've never had any problems.