The difference is that wanting sex (or anything else that isn’t another human being) affects only you. Having a child creates a new being who will be impacted, for better or for worse, and permanently, by the person or people introducing it to the world. Why someone wants to have a child is a valid question for that reason. It’s something anyone planning to have one should at least think about. And the next question should be, “Have you thought about what it might mean, in a practical sense, to be a parent for the rest of your life? Have you considered at least five ‘what-if’s?” If your desire to have a child affected ONLY you, it wouldn’t matter. But once you have one, it affects every moment of another person’s life.