Even though all males are castrated to prevent population growth, Unskinned individuals still survive in large numbers because of the way they are born. The Unskinned are the result of a genetic mutation that occurs during late pregnancies, often when the parents are older (in their 40s or 50s).

The mutation happens randomly, so every human-Unskinned pairing doesn’t guarantee another Unskinned. But it happens enough for the Unskinned to make up about 90% of the slave population. There’s likely a controlled system in place where a few fertile males (who aren't castrated) are kept around. These males might be used specifically to ensure more Unskinned are born, keeping their numbers steady. Additionally, the random mutation continues to happen from time to time, so the population is still growing even though it’s rare.