People of the nations of Moav and Ammon are blacklisted insofar as they are excluded from entering the Congregation of G-d (Devarim 23:3-4).
The Midrash notes how King David was "found" in Sedom (Bereishis Rabbah 50:10). In the immediate aftermath of Sedom's destruction, Lot's two daughters consorted with their drunken father to conceive Moav and Ammon (Bereishis 19:37). Here were the origins of David as he was a direct descendant of Ruth, the princess from Moav who married Boaz (See Book of Ruth).
The children of Moav and Ammon were barred from integrating into the Jewish people on account of these nations – children of Lot known for his hospitality to guests – failing to offer food and drink to the Chosen nation in their trek in the wilderness. Additionally, Balak the king of Moav employed Bilaam to curse the chosen people even though they posed no threat to his kingdom (Devarim 23:3-4).
The exclusion of Moav and Ammon from "entering the Congregation of G-d" (although they can convert to Judaism– See Rambam, Hilchos Issurei Bi’ah 12:17) was not an absolute restriction though. Albeit inconsistent with the usual principles of biblical exegesis, the law given to Moshe at Sinai (halachah l'Moshe meSinai) asserted that women proselytes were accepted and could marry into the Jewish fold (Yevamos 76b). Thus, Ruth - a woman from Moav - was able to embrace Judaism and, in fact, gave birth to royalty.
The Maharal (Netzach Yisrael Ch.32) gives a beautiful, conceptual explanation for why only the men from these two nations were excluded but not women. Why the discrimination?
The world is comprised of two categories: (1) tzurah, "form" and (2) chomer, "matter". The former relates to the 'purpose' or 'function' that shapes the latter. A piece of wood, a typical example of the latter, is comparable to "matter" until it is constructed into a table or chair which gives it its requisite "form" and function.
The all-encompassing tzurah-and-chomer model, where "matter" is given purposeful "form", describes, amongst other examples, the relationship between G-d and the universe, the soul and the body in a human being, potential and actual, the Jewish people and the gentiles, and man and woman as husband and wife.
Once "matter" assumes a particular "form" it cannot then easily move to assume another "form". Thus, a wooden table cannot easily be transformed into a chair. However where something is related to "matter" it stands ready to be fashioned into whatever "form" is deemed suitable.
Offspring of Lot and his daughters, Moav and Ammon are conceptually over-related to tzurah, "form". That is, their mother was formed from their father, and she consorted once again with their father. So in all respects, the daughter/wife was subordinate to her father/husband. Therefore Moav and Ammon strongly personify the male symbol of tzurah, "form".
The alien "form" of the resulting two nations was ideologically incompatible with the "form" and spiritual makeup of the Jewish nation. Moav and Ammon did not personify the "form" of the chosen people and their hospitality and kedushah, sanctity. On the contrary, these two nations woefully failed to offer food and drink to the wearisome Children of Israel in the desert. Moreover, after Bilaam's attempts to curse them backfired, Moav dispatched her maidens to lure the Jews into harlotry at the debacle of Shittim.
Tragically, the conceptual affinity of male members of Moav and Ammon to their national "form" could not be shed. Thus, there would be no opportunity for their men (themselves symbolic of "form") to undo their "form" and to assume the holy "form" of the Jewish nation by marrying into the faith.
But the women of Moav and Ammon could.
This is because their femininity is intrinsically related to the concept of "matter". In the same way, a married woman leaves her parents' home and upbringing to embrace the new identity, name and "form" of her husband, similarly, a woman from Moav such as Ruth was able to take on the new Jewish "form".
More than that, Ruth herself became "mother of royalty" whose progeny would lead to the birth of Mashiach, born of the royal Davidic lineage. She would give the "matter" of this world its true, perfected "form" in the Messianic Era where the glory of G-d will be universally acknowledged by Mashiach the son of David.
The course material is presented by Osher Chaim Levene, author of "Set in Stone: The Meaning of Mitzvah Observance" (Targum/Feldheim), a writer and educator in London whose website www.mitzva.org explores the wisdom of the commandments.