Spoilers for third section of Seveneves:
Moira is able to synthesize a Y chromosome to create boys for the second generation of infants on the Cradle. And yet, there is no explanation of what prevented racial mixing between the grandchildren of different Eves. If each Eve and her offspring were physically separated, this might make sense. But look at the (later) attraction of Kathtwo and Beled, and imagine a world where frequent sexual reproduction and avoiding inbreeding were absolutely essential to the survival of the human species.
In such a scenario, wouldn't the Eves's grandchildren be getting it on constantly? In such a scenario, I can't understand how racial purity would have been preserved, and certainly not how it would continue for 5000 years. For example, look how much black blood has intermixed with whites (and that's just over a few hundred years, not 5000). Or, for examples of enemies's children getting together, read Romeo and Juliet.
In story, I feel like Stephenson needed some sort of absolute prohibition on sexual intercourse, so that all babies would be generated via IVF (maybe to combat solar radiation?) in order to hold out the possibility that racial purity could be maintained.
Out of universe, I think that Stephenson just enjoyed his different archetypes of women enough that he figured it would be more fun to project them forward 5000 years, even if there's no possible way that such racial separation could have been achieved.
Is there any in-universe explanation which would justify the extended racial separation of the Seveneve's bloodlines?