Octavia E. Butler
This is book two of the Xenogenesis trilogy. A few humans have been rescued from a nuclear holocaust, and now have an opportunity to thrive—if they are willing to merge their DNA with their rescuers. The main character is Akin, a ‘human’ boy, who is much more than he appears. Kidnapped in infancy, he is taken by ‘pure’ human resisters who refuse to believe their rescuers. Rendered sterile by the Oankali, they are barely surviving and can have no children of their own.
The Oankali choose to leave Akin with the humans, even though they have the power to return him to his parents. They have decided to leave the choice of whether to return fertility to the resisters, or let them die out. If they regain their fertility, they will destroy themselves again.
How Akin deals with the burden of the decision forms the bulk of this story.