What is Zimri's sin?

Question

Note: This is Zimri that was speared by Phinehas for sleeping with a foreigner.

Wikipedia says that he committed idolatry. However, the Bible only mentions that he slept with a foreign woman. Nothing in Exodus seems to suggest he worshiped a foreign god.

What am I missing?

Looks like he just picked the wrong date or something.

Note: I didn't see the law in Torah prohibiting Israelites from hanging out with foreign women. There could be some vague things against marrying non-Jews, but there is nothing that says you can't hire foreign prostitutes (or date them), for example.

Answer 1

Some people were already being enticed to idolatry via their Moabite girlfriends (Num 25), so idolatry seems to sometimes be intertwined with relations.

More specifically, Sanhedrin 82a brings the following (Soncino translation):

And Moses said unto the judges of Israel, Slay ye every one of his men that were joined unto Baal Peor. Thereupon the tribe of Simeon went unto Zimri ben Salu and said unto him, ‘Behold, capital punishment is being meted out, yet you sit silent [i.e., inactive].’ What did he do? He arose and assembled twenty-four thousand Israelites and went unto Cozbi, and said unto her, ‘Surrender thyself unto me.’ She replied, ‘I am a king's daughter, and thus hath my father instructed me, "Thou shalt yield only to their greatest man". ‘I too,’ he replied, ‘am the prince of a tribe; moreover, my tribe is greater than his [Moses], for mine is second in birth, whilst his is third.’ He then seized her by her coiffure and brought her before Moses. ‘Son of Amram,’ exclaimed he, ‘is this woman forbidden or permitted? And should you say. "She is forbidden", who permitted thee Jethro's daughter’? At that moment Moses forgot the halachah [concerning intimacy with a heathen woman], and all the people burst into tears; hence it is written, and they were weeping before the door of the tabernacle of the congregation. And it is also written, And Phineas, the son of Eleazar, the son of Aaron the priest, saw it. Now, what did he see? — Rab said: He saw what was happening and remembered the halachah, and said to him, ‘O great-uncle! did you not teach us this on thy descent from Mount Sinai: He who cohabits with a heathen woman is punished by zealots?’ He replied. ‘He who reads the letter, let him be the agent [to carry out its instructions]’.

According to this source, Zimri not only sought his own pleasure but sought to embarrass and perhaps displace Moshe and, by extension, the leadership structure put in place by God. When Korach tried this he was killed; so too was Zimri.

Answer 2

The Rambam (Maimonides) writes with regards to a man marrying a non-Jewish woman:

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. 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."

Law 4 discusses the law of living with a non-Jewish woman (in public) without being married:

Whenever a man has relations with a gentile woman in public, i.e., the relations are carried out in the presence of ten or more Jews, if a zealous person strikes him and kills him, he is considered praiseworthy and ardent. [This applies whether the relations were] in the context of marriage or licentious in nature. This matter is a halachah conveyed to Moshe at Sinai. Support for this can be derived from Pinchas' slaying of Zimri.