Halakhah for Sanhedrin 162:17

He prohibited us from marrying heretics. And that is His, may He be exalted, saying, "And you shall not marry them" (Deuteronomy 7:3). And he explained what marriage is - "do not give your daughters to their sons or take their daughters for your sons." And in the explanation (Avodah Zarah 36b), they said, "The Torah forbade [it when it is] by way of marriage." And there are distinctions about the punishment of one who transgresses this negative commandment. And that is that when the one who has sexual relations with an Aramean has sexual relations in public - anyone who kills him while he is clinging to the sin has carried out the punishment, as Pinchas did to Zimri (Numbers 25:6-8). And they said (Sanhedrin 81a), "One who has sexual relations with an Aramean, zealots may attack him" - but with the conditions that we mentioned. And that is that he has relations with her in public, and [that the punishment is meted out] at the time of the act - like the story that happened (with Pinchas and Zimri). But if he did not do this in public or he [already] separated and the zealots did not attack him, he is liable for excision. However this excision is not made clear in the Torah: They said, "[When] zealots did not attack him, what is [the law]?" And it is explained that it is excision (cutting off), from His saying, "for Judah has profaned what is holy to the Lord - what He loves - and espoused daughters of alien gods. The Lord will cut off from the man that does this all living offspring" (Malachi 2:11-12). [This] implies that it is with excision. However when it becomes confirmed about a man - with witnesses and a warning - that he had sexual relations with an Aramean, he is lashed, by Torah law. And know this. (See Parashat Vaetchanan; Mishneh Torah, Forbidden Intercourse 12.)