(Mishnah): There is a fine for (raping or seducing) any of the following Na'aros: a Mamzeres, a Nesinah (a convert from the seven Kena'ani nations)...
29b (Beraisa - Shimon ha'Temani): "She will be his wife" refers to a woman with whom Kidushin is Tofes (takes effect);
R. Shimon ben Menasiya says, it refers to a woman whom he may keep.
These Tana'im argue about a Mamzeres or Nesinah (Chayavei Lavin). Kidushin is Tofes with them, but a Yisrael may not stay married to them.
According to R. Akiva, who says that Ein Kidushin Tofsin b'Chayavei Lavin, they argue about a Kohen Gadol who raped a widow.
Yevamos 76a - Question: May a Petzu'a Daka (castrated man, who may not marry a Yisraelis) Kohen marry a convert or freed slave?
If he has Kedushah (of Kehunah), he is forbidden. If not, he is permitted.
Answer (Rav Sheshes - Beraisa): A Petzu'a Daka Yisrael may marry a Nesinah.
This shows that he lacks Kedushah (of Kehal Yisrael). If not, he would transgress "Do not intermarry with them"!
Question (Rava): That Isur does not depend on Kedushah. It is because their children might serve idolatry!
The Isur is only while they are Nochrim. After conversion the Torah permits them, but Chachamim decreed against them. Chachamim forbade only people who can have children.
Objection: A Mamzer can have children. Why may he marry a Nesinah?
(Mishnah): Mamzerim and Nesinim may intermarry.
Retraction (of 8:i): Rather, Chachamim forbade only people of Kosher lineage.
Retraction (of 8 - Rava): "Do not intermarry with them" applies only after conversion. Before this, marriage does not take effect!
78b (Mishnah): Mamzerim and Nesinim are forbidden forever, both males and females.
(Rav Chana bar Ada): David decreed against Nesinim - "The king ... said that the Giv'onim are not from Benei Yisrael." (They are called Nesinim because they were given to be slaves to cut wood and draw water.)
79a - Question: David did not decree against the Nesinim. Moshe did - "From the woodchopper to the one who draws your water"!
Answer: Moshe decreed only for his generation. David decreed for all generations.
Question: Yehoshua decreed on them before David! "Yehoshua made them woodchoppers and water drawers for the congregation and the Mizbe'ach."
Answer: Yehoshua decreed only for when there is a Mikdash. David decreed even for when there is no Mikdash.
In Rebbi's day, they wanted to permit the Nesinim.
Rebbi: We can permit our share, but who will permit the share of the Mizbe'ach?
This argues with R. Chiya bar Aba.
(R. Chiya bar Aba): Yisrael's share is always forbidden. The Mizbe'ach's share is forbidden only when there is a Mikdash.
Rif and Rosh (Yevamos 24b and 8:4): Mamzerim and Nesinim are forbidden forever, both males and females.
Ba'al ha'Ma'or: We conclude that Moshe, David and Yehoshua decreed to forbid the Nesinim. This supports Rava, who forbids them before conversion mid'Oraisa, and after conversion only mid'Rabanan. Even though Rava retracted, we conclude like his original opinion. Therefore, a Petzu'a Daka Kohen may not marry a convert or freed slave, for perhaps he has Kedushas Kehunah. We cannot learn from the Heter of a Petzu'a Daka Yisrael to a Nesinah. The Rif did not discuss this.
Rebuttal (Milchamos Hash-m): This is like the simple reading of the Gemara. However, in many places the Gemara calls 'Nesinah l'Yisrael' Chayavei Lavin, e.g. regarding the fine of a rapist, Isur Kedushah and Isur Mitzvah. R. Yosi says that this disqualifies one from the army, even though Isurim mid'Rabanan do not. The Isur of a Chalutzah to a Kohen is mid'Rabanan. The Gemara often teaches it with Chayavei Lavin, but this is because it is learned from (an Asmachta from) a divorcee, and they are similar. If Nesinah were mid'Rabanan, there would be no reason for Chachamim to teach it with Mamzeres and make it like mid'Oraisa regarding disqualifying a Bas Yisrael and lashes. The Gemara taught that one is not liable twice for a divorcee and a Chalutzah. It did not say so about a Mamzeres and a Nesinah. Rav Sheshes, Rabah and Rav Yosef say that a Nesinah has a Kesuvah, because the Isur is mid'Oraisa. We cannot rule against them without a solid proof! The Gemara said that an Isah is a family without Safek Isurim mid'Oraisa, e.g. Mamzerim or Nesinim. It seems that an actual Giv'oni convert is forbidden mid'Oraisa, but future generations that were born in Kedushah are permitted mid'Oraisa. The Torah did not say how many generations are forbidden, nor does it say 'forever'. Only the converts themselves are forbidden, for their deeds were corrupt. David decreed for all generations. In Rebbi's day, they wanted to cease the slavery imposed on the Nesinim in order to permit them. This is not uprooting the decree of a previous Beis Din. Rather, it is like one who frees his slave. Rebbi answered that they are also slaves of the Mikdash, and we cannot permit this. Whenever the Gemara mentions Nesinim, it refers to one of the seven nations who converted, e.g. a Girgashi.
Note: The Girgashi fled to Africa. The Giv'onim already converted in the days of Moshe, and the other Kena'ani nations died in war.)
Nimukei Yosef (DH Masnisin): Since the Torah did not specify how many generations are forbidden, we learn from Mamzerim to be stringent (all generations are forbidden).
Question: Rava said that "Lo Sischaten Bam" refers to before conversion. He retracted to say that it refers to after conversion because there cannot be marriage with Nochrim. In Avodah Zarah we say that any Nochris is forbidden (before conversion) mid'Oraisa due to "Lo Sischaten Bam"!
Answer (Milchamos Hash-m): It is unreasonable that the Torah would say "Sischaten" if the Isur were only before conversion, when there cannot be marriage. It is reasonable that the verse discusses the seven nations after conversion, and we include all Nochriyos before conversion.
Rambam (Hilchos Isurei Bi'ah 12:22): One who converted from the seven Kena'ani nations is not forbidden mid'Oraisa from marrying a Yisrael. It is known that of the seven nations, only the Giv'onim converted. Yehoshua decreed that their males and females may not marry Yisraelim. He forbade them only when there is a Mikdash - "V'Chotvei Etzim v'Sho'avei Mayim l'Veis Elokai." He attributed the Isur to the Mikdash.
Magid Mishneh: The Rambam rules like the Sugya in Yevamos (78b), which forbids Nesinim mid'Rabanan. He holds that it argues with Rava (76a), who forbids mid'Oraisa.
Rambam (23): They are called 'Nesinim' because they were given to serve the Mikdash. David decreed that they may never enter Yisrael, even when there is no Mikdash. It says in Ezra "U'Min ha'Nesinim she'Nosan David veha'Sarim la'Avodas ha'Leviyim." The Isur is not attributed to the Mikdash.
Shulchan Aruch (EH 4:1): Mamzerim and Nesinim are forbidden forever for all generations, both males and females.
Beis Shmuel (1): R. Tam holds that the Torah forbids Nesinim after conversion. Chachamim decreed slavery.
Shulchan Aruch (5): If a Yisrael fathered a child from any of the forbidden nations, the child is like her. If a Yisraelis had a child from any of the forbidden nations, the child may marry a Yisrael, but is disqualified from Kehunah.
Chelkas Mechokek (2): This refers to the forbidden nations before conversion.