Sefer HaChinuch ספר החינוך

Not to distance an Edomite of the third generation from coming into the congregation: Not to distance the seed of Esav after they convert — which is to say that we should not prevent ourselves from marrying with them, as there is no prohibition, like there is with Ammon and Moav; but rather, they are permissible after two generations. And about this is it stated (Deuteronomy 23:8-9), “You shall not despise an Edomite, since he is your brother, etc. Children that will be born to them in the third generation shall come into the Congregation of the Lord.” And the third generation is the grandson of the convert. And they, may their memory be blessed, said (Yevamot 78a) about a pregnant Egyptian woman that converted while she is still pregnant, that her son is called the second [generation], even though his conception was while the mother was still an Egyptian.

The root of this commandment is revealed — that it is to inform us that we should not treat the seed of Esav [as] prohibited and that we should not distance them via an a fortiori argument (kal ve’chomer) from the Children of Ammon and Moav: to say that [since] God distanced us from Ammon and Moav because of one time that they did not greet us with bread and water, is it not all the more so with the Edomites and the Egyptians who caused so much pain to Israel? Hence, this warning about them came to us that we should not prevent ourselves from marrying them. Since the subjugation that they subjugated us was a decree upon us from God, it is not fitting that we fix a hatred in our hearts about it. And they are called our brothers in their converting and coming to be shielded under the wings of the Divine Presence. But with Ammon and Moav, there is another reason for hating them, and that is the matter of the great villainy that they found in their hearts to do — everything is as I wrote above adjacently (Sefer HaChinukh 561).

From the laws of the commandment is that which they, may their memory be blessed, said (Yevamot 78a) [that if] an Egyptian marries [another] Egyptian and she converts while she is still pregnant, the offspring is called the second [generation], as it states, “Children that will be born to them, etc.” — and the verse makes it dependent on the birth; and [that if] an Ammonite convert marries an Egyptian woman, the offspring is an Ammonite, [but in the case of] an Egyptian convert that married an Ammonite woman, the offspring is an Egyptian. This is the general principle: With the [gentile] peoples alone, it goes after the males, and like we find that Scripture relates them to the males, as it is stated (II Kings 20:12), Baladan the son of Baladan. [But if] they [both] convert, it goes according to the less [advantageous]. 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.)

This prohibition that we are prevented from distancing them after they have converted is practiced in every place and at all times by males and females. And one who transgresses it, and decides in his mind not to marry into them — even after two generations — from the angle that they troubled Israel or from the angle that he has fixed a hatred in his heart for them because they came from another people, has violated this negative commandment. And we do not give lashes for it, since there is no act [involved] with it.