Part of me wants there to be a set of standards for a person to have the right have a child. The thing is having offspring has been such a huge part of human existence, its just been always thought of as default right of person. Creating a legal structure for this wouldn't be impossible, and it would, and could only be applied to extreme cases. I mean human society already has a set of rules for nearly everything, including for the a person' conduct after they have children, and getting rid of a child you don't want. As mentioned in this thread already, China, made a legal regime to regulate its citizens rights on having children, out of the necessity of overpopulation. So China, didn't really tell anyone that cannot have a child, they just said how many you can have.
