Some (like myself), believe that this is just a very facetious way of saying "no, you will never have children." Not every prophecy is meant to be fulfilled, some are meant to influence the way the characters think of themselves and their lives, and are realized in symbolic or metaphorical ways. Daenerys wants to be a mother, but her goals in this part of her life are much more concrete: She wants to reclaim the Iron Throne. Only then, and she said in "Beyond The Wall", will she consider her successor.