This part of the plot is kind of why I wanted to release all at once. Toph's infertility is solved by the end of Part 3, but I don't go into what made her infertile in Part 3. That particular mystery will be revealed in Part 4. However, I'm going to give you a minor spoiler....
This isn't Sokka's fault. The Toph in the story doesn't know but Granny Toph (the one telling the story) does know.
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