In the ancient near east, a eunuch was a male who was unwilling or unable to marry and bear children. In popular culture, this has been reduced to the stereotype of a castrated male, but the original understanding includes males with intact genitals. The castrated males accounts for the unable aspect of marriage and procreation: castration prevents (natural) erection and ejaculation which prevents childbearing and no family would give their daughter for marriage to someone who could not consummate marriage. Indeed, a lack of consummation, even in the strict Roman Catholic doctrine on marriage, prevents a marriage from being official. For this reason, eunuchs were commonly given domestic authority over the female members of a royal house: wives, daughters, and concubines. The king needed the utmost trust that their wives would not be raped or unfaithful or that the royal lineage be dubious.
Mehu'man, Biztha, Harbo'na, Bigtha and Abag'tha, Zethar and Carkas, the
seven eunuchs who served King Ahasu-e'rus as chamberlains. (Esther 1:10, RSV)
The Identity of Un-castrated Eunuchs - Eunuchs included people we now consider transgender
God's Promise for Eunuchs - What Scriptures says about eunuchs and trans people directly
Trans Saints - including three little-known protagonists in Scripture who were eunuchs