Sanhedrin 81b - Jew Having Relations with a Goy

R. Yakov Kaminetzky (Parshas Vayechi) tries to explain the difference between the kana'us of Shimon and Levi. They were both involved in killing out shechem, and were chastised by Yakov. Levi learned how to control their kana'us and use the Torah to moderate when it is appropriate. Shimon on the other hand never controlled his kana'us. In the episode with Zimri, it was the Torah controlled kana'us of Pinchas (shevet levi) who killed the uncontrolled behavior of Zimri who was from Shimon. His point is that the din of הבועל ארמית קנאין פוגעין בו, is exactly that. As Rashi writes, it is a halacha reserved for בני אדם כשרין המתקנאין קנאתו של מקום, not a more hotheaded approach to venting personal anger.

R. Yakov illustrates the idea of an uncontrolled kana'aus that is not within the confines of Torah. The question is sometimes asked, which is worse - to marry a goy or marry a woman who won't keep hilchos nidah? The kana'us of a תלמיד שלא שימש כל צרכו would surely be to respond based on the associated punishment - relations with a nidah is kareis, therefore worse. However, R. Yakov points to the Rambam (Hilchos Issurei Bi'ah 12:7-8) who writes about the severity of even having relations with a goy.

The Rambam writes:

עון זה, אע"פ שאין בו מיתת ב"ד אל יהיה קל בעיניך, אלא יש בו הפסד שאין בכל העריות כמותו, שהבן מן הערוה בנו הוא לכל דבר ובכלל ישראל נחשב אע"פ שהוא ממזר, והבן מן הכותית אינו בנו וכו' ודבר זה גורם להדבק בעכו"ם שהבדילנו הקב"ה מהם ולשוב מאחרי ה' ולמעול בו

The Rambam points to the child born from the illegal relationship as a more accurate litmus test to the severity of the relationship. A child born from adultery is a mamzer, but retains status of a Jew, whereas a child born to a non-jewish woman from a Jewish man doesn't even retain the status of a Jew. Furthermore, marriage out of the faith leads to the deterioration of the very fabric that holds the Jewish people together by causing attachment to the goyim and reject Hashem. Based on this Rambam, Rav Yakov writes that it is obvious that it is better to marry a Jewish woman and violate an issur kareis regularly, than to marry a non-jew.

A very strong proof the severity of the issur to have relations with a goy is the Nimukei Yosef at the end of perek ben sorer u'moreh. In the context of giving up one's life to avoid prohibited sexual relationships, the Nimukei Yosef writes:

מיהו נכרית דאנסה ליה לישראל לבא עליה יהרג ואל יעבור שזו ודאי בכלל עריות חמורות היא, דהא זימנין דחייב מיתה כגון בפרהסיא וכמעשה שהיה דקנאין פוגעין בו, ואם לא פגעו בו קנאין חייב כרת, הלכך אפילו בצנעא דליכא כרת יהרג ואל יעבור מדין אבזרהא דהא אשה זו פעמים שהיא ערוה גמורה וחייב עליה כרת

The Nimukei Yosef says that one would be obligated to sacrifice their life to avoid having relations with a goy. Even in a situation where one isn't able to be killed, such as in private, they must give up their life to avoid the prohibition.