This was asked once before! My answer was not very satisfactory though, but it's the only thing I've ever read that approached the question. I'd caution you not to make too much of the concubines and sexual access though. Sterility is a red herring! Eunuchs are more than castration! So don't automatically assume sexual control = eunuchs and eunuchs = sexual control, because as you can see, it doesn't work as a cultural universal!