Gemara

Avodah Zarah 36b (Beili citing Rav): Nochri girls are among the 18 decrees.

Question: Their daughters are forbidden mid'Oraisa - "v'Lo Sischaten Bam"!

Answer #1: Mid'Oraisa, only the seven Kena'ani nations are forbidden. They (Beis Shamai and Beis Hillel) decreed against all other Nochrim.

Question: According to R. Shimon, "Ki Yasir Es Bincha me'Acharai" includes all Nochrim (all would veer a Yisrael away from Hash-m). How can we answer?

Answer #2: Mid'Oraisa, one may not live with a Nochris Derech Ishus (like one lives with a wife. They decreed against Zenus (extramarital relations).

Question: The Beis Din of Shem already decreed against Zenus of Yisraelim with Nochrim - "Hotzi'uha v'Sisaref" (Yehudah was about to kill Tamar for this)!

Answer: The Torah (i.e. the allusion to Shem's decree) forbids Zenus of a Yisraelis with a Nochri, lest she will be drawn after him. They (Beis Shamai and Beis Hillel) decreed against Zenus of a Yisrael with a Nochris.

Question: A tradition from Sinai forbids Zenus of a Yisraelis with a Nochris! If a Yisrael has Bi'ah with a Nochris, zealots may kill him (during the act).

Answer: That is only if the Bi'ah is in public; like when Pinchas killed Zimri. They decreed even against Zenus in private.

Question: Rav Dimi taught that the Beis Din of Chashmona'im already decreed against such Bi'ah in private! One who does so is liable on four counts, as if he had Bi'ah with a Nidah, slave, (Nisu'in with a) Nochris and a married woman. Ravin obligates for Bi'ah with a Nidah, slave, Nochris and Zonah.

Answer: The Chashmona'im forbade Bi'ah. Beis Shamai and Beis Hillel forbade seclusion.

Berachos 58a: Rav Shila lashed a man who had relations with a Mitzris.

Eruvin 19a: Those sentenced to Gehinom go briefly, and Avraham takes them out, except for a Yisrael who had relations with a Nochris. The foreskin of such a man stretches, and he does not appear like a Yisrael to Avraham.

Rishonim

Rambam (Hilchos Isurei Bi'ah 12:1): If a Yisrael had Bi'ah with a Nochris from other nations, or vice-versa, Derech Ishus, they are lashed mid'Oraisa - "v'Lo Sischaten Bam. Do not give your daughter to his son, and do not take his daughter for your son." This applies to the seven and all nations. Ezra said "we will not give our daughters to Amei ha'Aretz, or take their daughters for our sons."

Magid Mishneh: Rava said that marriage applies only after conversion. The Meforshim explain that the Torah forbids anything Derech Ishus.

Rambam (2): The Torah forbids only Derech Ishus. One is lashed mid'Rabanan for Bi'as Zenus with a Nochris. This is a decree lest come to marry. If he designated her for Zenus he is liable for Nidah, a slave, Nochris and Zonah. If he did not designate her, just happened to had Bi'ah with her, he is liable only for a Nochris. All these liabilities are mid'Rabanan.

Magid Mishneh: Berachos 58a teaches that one is lashed for relations with a Nochris. The Rambam's text in Avodah Zarah says that Beis Din of Chashmona'im decreed about Bi'ah in private when he designated her.

Rambam (3): This refers to a Yisrael. A Kohen who has Bi'ah with a Nochris is lashed mid'Oraisa for Zonah. There is no difference between a Nochris Zonah and a Yisraelis Zonah. He is lashed for Bi'ah alone, for there is no Kidushin.

Poskim

Shulchan Aruch (EH 16:1): If a Yisrael had Bi'ah with a Nochris, or vice-versa, Derech Ishus, they are lashed mid'Oraisa - " v'Lo Sischaten Bam."

Rema: Some disagree.

Beis Shmuel (1): The Rambam holds that one is lashed only for Bi'ah Derech Ishus, but not for a mere marriage (ceremony) or Bi'ah alone. Other Poskim obligate for "do not take" for Bi'ah alone with a Nochris, and for "v'Lo Sischaten" for Kidushin (with the seven nations), even without Bi'ah.

Question (Bach DH u'Mah she'Chosav v'Im): The Tur says that if he designated her for Zenus, he is lashed mid'Rabanan. This implies that if he designated her for marriage, he would be lashed mid'Oraisa. Later, he rules like Chachamim, who exempt for the other nations!

Answer (Bach): The Tur first wrote what all agree to (if he designated her for Zenus, he is lashed mid'Rabanan). Later, he brings the Rambam's opinion about Derech Ishus, and he disagrees.

Bach (DH u'Mah she'Chosav Derech): The Torah needed to permit a Yafes To'ar even if she is married. This shows that marriage applies to a Nochris, even though she has no Kidushin. This is why the Tur obligates for a Nochris only for Bi'ah, even though for a convert of the seven nations one is liable only for Kidushin.

Shulchan Aruch (ibid.): One who had Bi'as Zenus with a Nochris is liable mid'Rabanan for a Nochris and Zonah, and he is lashed mid'Rabanan.

Chelkas Mechokek (2): The Gemara and Rambam omit Zonah.

Gra (4): This is a textual error in the Shulchan Aruch.

Shulchan Aruch (ibid.): If he designated her for Zenus, he is liable mid'Rabanan for Nidah, slave, Nochris and Zonah. If he is a Kohen, he is lashed, even if he happened to had Bi'ah with her, he lashed mid'Oraisa for Zonah.

Chelkas Mechokek (3): Tosfos says that the Isur Zonah is a decree if he is a Kohen. The Rambam forbids a Kohen mid'Oraisa; the decree was for a Yisrael. Tosfos holds that a Zonah is only if we know that she was Mezanah.

Taz (2): The Semag calls Bi'as Nochris 'the most severe of the Arayos'. He did not bring that such people do not leave Gehinom. Mahara Stein says that in truth he comes out after 12 months, like one who transgressed Eshes Ish. The Maharshal says that it is no harsher than Arayos of Kares and Misas Beis Din. Rather, there are great losses; one will be led to do idolatry and be Mechalel Hash-m. Tosfos says that one does not descend to Gehinom for this itself. Rather, if one descends for other Aveiros, this causes that he not ascend. I say that Semag came to answer a question, not to teach about punishment. We say that he fathers a son for idolatry; elsewhere we say that it is not his son! One fulfills Peru u'Rvu even though a Mamzer, but not through a Nochris' child. It is not his son to benefit him. We are stringent to say that it is his son only for detriment, i.e. that he fathered a son to idolatry.

SMA (CM 34:4): The Mordechai (Sanhedrin 795) holds that one who had Bi'ah with a Nochris is Pasul mid'Rabanan for testimony. The Beis Yosef connotes that he is Pasul mid'Oraisa.