Taking into account your other post, I don't see this so much as a double standard, but as her expressing her fears. Was she expressing her fears, or was she telling you can't go and visit this girl? Did she expect you would leave her when you found out about her affair? You didn't, but has she been waiting for "the other shoe to drop"? Your new girl has expressed difficulty accepting being in a poly relationship. Your wife maybe affraid you were just waiting to leave until you found someone to replace her.
Other than having honest communication with her and assuring her that leaving is not in your plans (which it looks like you have already done), there isn't much you can do aside from proving it. This is really hers to deal with.