So this is my therory.
Does anyone remember the spirit kho in the North Pole part 2? well remember how a previous Avatar got all angry at this spirit for steeling the face of the woman he loved? well I don't think that the Avatar is ever supposed to marry a woman and procreate. think about it. the avatar must die to ensure that he is reincarnated into the next tribe. So that means he cannot have any children or Marry anyone for that matter because if he were to have children or a child with a person he loves then the child would or may have the avatar spirit enstilled within him. Hence why Kho or the Face-stealer stole the face of a woman a previous avatar loved. Plus this spirit did say that they will be meeting again...so yeah....not to crush the spirits of all those Aang and Katara couple fans but i think this is what may happen. But who knows maybe it won't just a theory.