Cedric and Bertrand want to see a movie. Bertrand wants to see Muscled Duded Blow Stuff Up. Cedric wants to see Quiet Remembrances: Time as Allegory. There's also Middlebrow Space Fantasy. They are rational but not selfish - they care about the other's happiness as much as their own. What should they see?
Sure. But it will be my intuition, and not some impersonal law. This means it's okay for me to want things like "there should be some puppies, but not too many," which makes perfect sense as a preference about the universe, but practically no sense in terms of population ethics.