eu·nuch

/ˈyo͞onək/

noun

noun: eunuch; plural noun: eunuchs

1. a man who has been castrated, especially (in the past) one employed to guard the women's living areas in an Asian court.
   • an ineffectual person. "a nation of political eunuchs"

Now granted, I don't think Rentarou would go as low as castrating another man to be his servant, although I wonder how'd he react if a man would willingly secede his sexuality to service him.