Sefer HaChinuch ספר החינוך

That a mamzer (a child born from a forbidden marriage) should not marry a daughter of Israel: That a mamzer is prevented from marrying a daughter of Israel. And about this is it stated (Deuteronomy 23:3), “No mamzer shall come into the congregation of the Lord,” meaning to say that he should not come to marry a woman from the daughters of the congregation of the Lord. But it is truly permissible for him to enter with them into all of their places of residence and to give and take with them in all things [just] like [any other] one of the children of Israel. And they, may their memory be blessed, have already said (Horayot 13a) that a mamzer Torah scholar precedes an ignoramus Kohen (priest) in the reading of the Torah. (See Rambam on Mishnah Gittin 5:7.)

About the roots of this commandment are that since the conception of the mamzer is very bad (see Tractate Kallah 1) — as it is done in impurity and with invalid thoughts and the counsel of sin — and there is no doubt that the nature of the father is hidden in the child, therefore God, in His kindness, distanced the holy seed (of the Jewish people) from him; [just] like He separated us and distanced us from all bad things. (See Guide for the Perplexed 3:49.)

From the laws of the commandment is that which they, may their memory be blessed, said (Yevamot 49a and see Mishneh Torah, Forbidden Intercourse 15:1) that the mamzer that the Torah forbade is one that is born from the sexual prohibitions mentioned in the Torah, except from a menstruant women — as a son from her is defective, but he is not called a mamzer. And whether it was under coercion or volitional, in any case, he is called a mamzer. And [if] a gentile or a slave has sexual relations with a daughter of Israel, the offspring is fitting (as opposed to being a mamzer) — whether she is single or whether she is the wife of a man, whether it is under coercion or whether it is volitional. But [if] a gentile or a slave has sexual relations with a mamzeret, the offspring is a mamzer, but if a mamzer has sexual relations with a gentile woman, the offspring is a gentile — and if he converts, he is fitting immediately, like all of the converts of the nations (Yevamot 45b and see Mishneh Torah, Forbidden Intercourse 15:3-4). And if he has sexual relations with a slave woman, the offspring is a slave. And if [his master] freed him, the offspring is fitting, like other freed slaves — and they are permitted to come (marry) into the congregation of Israel. This is the general principle: A son that comes from a slave woman or a gentile woman or from a slave or from a gentile or from a mamzer, the offspring is like [the woman] and we do not pay attention to the [identity of the] father at all. And because of this thing, the Sages permitted (Kiddushin 69a) a mamzer to marry a slave woman, in order to “purify” his children — as behold, he frees them and they are found to be free men. And a mamzer is permitted to marry a woman convert and a woman mamzer is permitted to marry a convert, as it states, “into the congregation of the Lord” — and the congregation of converts is not called a congregation — and [in these cases,] the offspring follows the defective one. (See Mishneh Torah, Forbidden Intercourse 15:7,9.) And even [if] a convert marries [another] convert, [their] son is permitted to [marry] a mamzeret, from the reason that we mentioned — even though his conception and his birth were in purity (after they converted). And [this is the case] even [for] his children and grandchildren until the name of gentileness is removed from them, meaning to say that it is not known that he is [descended from converts]. But [if] a convert marries a daughter of Israel or a son of Israel marries a convert, the offspring is an Israelite in all matters and forbidden to marry a mamzer woman. And the rest of the details of the commandment are elucidated in the eighth chapter of Yevamot and at the end of Kiddushin. (See Tur, Even HaEzer 4.)

And this prohibition is practiced in every place and at all times, by males and females. And one who transgresses it, and is a mamzer and marries a daughter of Israel — or [if] an Israelite marries a mamzeret — once he has sexual relations after the betrothal, they are lashed, both he and her. But [if] he had sexual relations and did not betroth her, they are not lashed — as you do not have anyone who is lashed for sexual relations without betrothal in all of the negative commandments of the Torah, except for a High Priest with a widow, as I have written in its place in the Order of Emor el HaKohanim (Sefer HaChinukh 273). This is the opinion of Rambam (Mishneh Torah, Laws of Forbidden Intercourse 15:7,9), but there are commentators that have written that they are lashed for sexual relations only, [even] without betrothal (Raavad on Mishneh Torah, Forbidden Intercourse 15:2).