Jesus taught that there are three types of eunuchs- those born so (by physical or mental defect who are unable to marry), those who are made eunuchs by men (such as Daniel and his three friends), and those who make themselves eunuchs for God (such as John the Baptist, Paul, and possibly Elijah). (See Matthew 19:12.)
That’s just rabbinical commentary on the commandment for eunuchs not to marry.