Tafsiran Bilangan 25:3: Rashi, Midrash Lekach Tov, Rashbam & Rabbeinu Bahya

פעור — It was so named because they bared (פער) their anus and relieved themselves before it: this was its ceremonial rite (Sanhedrin 60b).

And Israel joined themselves to Baal Peor." At first, they would enter modestly, and afterwards, they began to enter in pairs, like the coupling of cattle. Another interpretation: "And Israel joined themselves" means they were joined together like a bracelet on the hand. "To Baal Peor" refers to idol worship, where they would eat and drink before it and then defecate there. This is what the sages said: "One who throws a stone at Merkulis, this is his worship." One who defecates before Baal Peor, this is his worship. The rabbis taught: There was a story with a Jewish man who hired his donkey to a Canaanite woman. When she arrived at Peor, she said to him, "Wait until I go in and come out." After she came out, he also went in and defecated in front of it, wiping himself with a reed. The priests would ridicule him, saying, "There was never a person who served him like this." Rav Yehuda said that Rav Ma'aseya related a story about a Canaanite woman who fell ill and said, "If you stand by me in my illness, go and serve every foreign worship in the world." When she arrived at Peor, she asked the priests how they worshiped it. They told her to descend and drink wine and then defecate in front of it. She said, "It is better to return to my illness and not serve idolatry in this way." But Israel "joined themselves" to Baal Peor, and the anger of the Lord burned against Israel.

ויחר אף, the pestilence being proof of G’d’s anger.

ויחר אף ה' בישראל AND THE ANGER OF THE LORD WAS KINDLED AGAINST ISRAEL — he sent a plague among them (cf. v. 8).