Male sexuality starts with testosterone and ends in the head. Men who've had double orchidectomy (both balls removed) carry right on having sex, they just don't need to worry about contraception.
I digress, but... it amuses me when the Gor books use "eunuch" as the ultimate insult for men who aren't dominant enough. In old China the court ladies prefered eunuchs as lovers, and not just because they were sterile: they were reputedly able to fuck for hours.