הלכה אויף מלאכי 2:11

A man who has sexual relations with an idol worshiper, if zealots have not killed him and he has not received lashes at the hands of the court, his punishment is explicit in the words of tradition (Prophets), that he receives excision, as is written, “…For Judah has profaned the holiness of God which He loves, and has married the daughter of his strange god. May God cut off the man that does this, he that calls and he that answers…” (Malachi 2:11-12). He shall not have a caller among the wise nor a responder among the students. Note: This sin has a deficit which is not present among all the other sexual proscriptions, for the son derived from a (Canaanite) female slave or from a Kuthite (non-Jewish) female is not considered his son, as opposed to descendents from other sexually proscribed women (Tur in the name of Rambam). One who has sexual relations with an idol worshiper in public, whose law is that zealots kill him, as will be explained Choshen Mishpat chapter 425, is included in the classification of sexual proscriptions, and the law is that he should permit himself to be killed rather than transgress (Beit Yosef in the name of Orchot Chaim in the name of Rambam) as is the case with other sexual proscriptions and as explained in Yoreh Deah chapter 157.

From the laws of the commandment is that which they, may their memory be blessed, said (Sanhedrin 81a) that zealots would attack one who has sexual intercourse with an Aramean (gentile) publicly in the eyes of ten or more Israelites. And the proof of the thing is the story of Pinchas and Zimri. But the zealot is nonetheless only permitted to attack him at the time of the promiscuous act, and like in the story that happened; as it is stated (Numbers 25:8), “and the woman through her belly.” But if he separated [from her], we do not kill him, but [rather] bring him to the court and they administer lashes [upon] him, since he did the act publicly. [If] the zealots did not attack him and the court did not administer lashes [upon] him, we know from the words of tradition that he is [punished] by excision, as it is written (Malachi 2:11-12), “and he who husbands (read here as, “who has sexual intercourse with”) the daughter of a foreign god. The Lord will excise the man that does it.” And a gentile who has sexual relations with an Israelite — if she is a married woman, he is killed over her; but, if not, he is not killed. But a Jew who wantonly has sexual relations with a gentile woman — even [if it is] by way of harlotry — she is nonetheless killed, since a mishap happened to an Israelite through her, like the law of an animal. And this thing is explicit in the Torah, as it is stated (Numbers 31:16-17), “They were the ones that were with the word of Bilaam against the Children of Israel, etc. and any woman that could know a man sexually they killed.” [This] and the rest of its details are elucidated in Avodah Zarah and Yevamot and Kiddushin. (See Mishneh Torah, Laws of Forbidden Intercourse 2.)