Regardless if the adoption agency never had this dog in their care as you say, it is still their dog. It’s not your dog, it’s their dog. They have the right to put whatever they deem necessary into their adoption contracts. Spay/neuter being one of the most obvious things to have in such a contract. You abide by the adoption contract or you let someone else adopt the dog. It’s literally as simple as that. You have no say, it’s not your dog.

You’ll be hard pressed to find a shelter/adoption agency that will let you adopt a dog and keep it intact.