What is the prohibition of intermarriage?

Question

The Gemara says that there's no kiddushin between a Jew and a non-Jew (the marriage is not effective), yet the only way to violate the prohibition of intermarriage is through marriage (at least for those who thing Yi Avid Mahane. Sexual intercourse on itself is not marriage, may be only a rabbinic prohibition)! So what specific action is forbidden?

Answer 1

The Gemara (AZ 36b), concerning non-Jew women, made a clear difference between two kind of man-woman relationship: marital or non marital. Marital relationship is a natural reality which was anterior to Matan Tora and is not a Jewish particularity. See Rambam, Nashim, Yishut, 1, 1:

Before the Torah was given, when a man would meet a woman in the marketplace and he and she decided to marry, he would bring her home, conduct relations in private and thus make her his wife.

And halacha 4:

Before the Torah was given, when a man would meet a woman in the marketplace, and he and she desired, he could give her payment, engage in relations with her wherever they desired, and then depart. Such a woman is referred to as a harlot

You see here a natural delimitation between marital and non-marital behavior.

For the intermarriage, the Rambam stated in Kedusha, Isure Bia, 12, 1-2:

When a Jew engages in relations with a woman from other nations, [taking her] as his wife or a Jewess engages in relations with a non-Jew as his wife, they are punished by lashes, according to Scriptural Law.1 As [Deuteronomy 7:3] states: "You shall not intermarry with them. Do not give your daughter to his son, and do not take his daughter for your son." This prohibition applies equally to [individuals from] the seven [Canaanite] nations and all other gentiles. This was explicitly stated in Ezra [Nechemiah 10:31]: "That we will not give our daughters to the gentiles in the land and that we will not take their daughters for our sons."

The Scriptural prohibition applies only to marital relations.

The Shulchan Aruch ruled as Rambam in Even Haezer 16, 1

I hope that now the question is answered.

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Now we can enter a bit more deeply in the topic. In Gemara two opinions of Tanayim are exposed concerning the verse of intermarriage. The first opinion is from Rabbaban, the verse prohibits only to marry, within Jew marriage rules, a convert from the 7 Canaanite nations⁽¹⁾. The second, the opinion of Rabbi Shimm'on, is that the verse prohibits to marry (in the old, before Tora manner) someone non-Jew when he is not converted to Judaism. Rambam ruled as Rabbi Shim'on and the Smag (Lavin 112) and the Tur ruled as Rabbanan. One question is what is the opinion of Rabbi Shim'on for the case of a convert from 7 Canaanite peoples. The Sefer Hachinuch in Mitsva 427, which follows exactly the Rambam, ruled that for this detail, we (Rabbi Shim'on) agree with Rabanan and prohibits by this verse. So he, following the Rambam prohibits. Nowadays we can not consider anybody as from 7 peoples and to marry a convert is never prohibited by this verse (See also Shaar Hamelech on Rambam IB 12, 1, which explain this following the Ran in Rif AZ).

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⁽¹⁾ _{Gemara Yebamot 76a: Subsequently Raba stated: What I said is of no consequence. For while they are still idolaters their marriages are invalid; only when they are converted are their marriages valid. {Rashi: The verse mentione marriage, so it cannot concern their status before conversion because there is no [halachically] marriage with non-Jew, but obviously the verse talk about them after conversion ...}. See also in the Smag, lavin 112, in link above and in Sefer Hachinuch.}

Answer 2

Rambam addresses this in Hilkhot Issurei Biah (12:1-2):

א ישראל שבעל גויה משאר האומות, דרך אישות, או ישראלית שנבעלה לגוי, דרך אישות--הרי אלו לוקין מן התורה, שנאמר "לא תתחתן, בם" (דברים ז,ג): אחד שבעה עממין, ואחד כל האומות באיסור זה. וכן מפורש על ידי עזרא "ואשר לא ניתן בנותינו, לעמי הארץ; ואת בנותיהם, לא ניקח לבנינו" (נחמיה י,לא). ולא אסרה תורה, אלא דרך חתנות

ב אבל הבא על הגויה דרך זנות--מכין אותו מכת מרדות מדברי סופרים, גזירה שמא יבוא להתחתן.

1) A Jewish male who engages in relations with a non-Jewish female in a manner of marriage, or a Jewish woman who engages in intercourse with a non-Jew in the manner of marriage--these are subjected to whipping according to Biblical law, as it says "do not marry them" (Deut. 7:3). Both members of the 7 (Canaanite) nations, and members of all other nations are equivalent in this law. Ad this is also explicit in the words of Ezra "And that we we will not give our daughters to the nations of the land, and their daughters, we will not take for our sons" (Nehemiah 10:31). The Torah only forbade this in the manner of wedding.

2) However, one who engages in intercourse with a non-Jewish woman in the manner of a non-marital liaison, is subjected to rabbinically imposed lashes; as an enactment lest he come to get married. (Translation my own).

To summarize it is not halakhic marriage (which as you noted cannot exist) to a non-Jew which is Biblically forbidden, nor is it intercourse alone, but rather, intercourse in the manner of wedding and marriage. That is, the prohibition is to have intercourse in the social framework of a marital relationship; even though technically there is no marriage.

However, there is a separate rabbinic prohibition against sexual intercourse even outside of marriage.