Sefer HaChinuch ספר החינוך

That a eunuch not marry a daughter of Israel: That one whose sexual organs were damaged by the blow of a person or an animal or a tree — meaning to say, not at the hands of the Heavens — to the point that he cannot reproduce is prevented from marrying a daughter of Israel. And about this is it stated (Deuteronomy 23:1), “No one whose testes are crushed or whose member is cut off shall come into the congregation of the Lord.”

About the roots of this commandment are that [one] distance himself [from this] so that he will not damage [his] reproductive organs in any way. (See Guide for the Perplexed 3:49.) As is known, there are kings that castrate males to appoint them as keepers of women and there are lowly males who want this thing in order to earn the king’s table and to benefit monetarily. And we, the holy people — in our knowledge that any eunuch caused by human means will be disqualified from joining any more with a daughter of Israel or to establish a home or the way of marriage — will distance this matter and it will repulse us. And with this argument, we find an explanation for the distinction in the prohibition between that which is damaged by human means and that which is damaged by the hand of the Heavens.

From the laws of the commandment is that which they, may their memory be blessed, explained (Yevamot 70a, and see Mishneh Torah, Forbidden Intercourse 16:3), “Who is [considered] ‘one whose testes are crushed?’ Anyone whose testicles were wounded [...]. And [who is considered] ‘one whose member is cut off?’ One whose penis was cut off.” And there are three organs that the male reproduces with: with the penis, and with the testicles and with the paths in which the seed matures — and they are called the testicular ducts. And therefore from when one of these three organs is wounded or cut or crushed, behold, he is disqualified. And the rest of its details are elucidated in the eighth chapter of Yevamot.

And this prohibition is practiced in every place and at all times, by males. And one who transgresses it — and he is one whose testes are crushed or whose member is cut off — and marries a daughter of Israel and has sexual relations with her, is liable for lashes. But he is permitted to marry a convert or a freed slave. And there are some commentators that have said that he transgresses with sexual relations alone, [even] without marriage, and so is it in the Talmud Yerushalmi.