Charles Ellicott Commentary Psalms 106:28-31

"They joined themselves also unto Baal-peor, And ate the sacrifices of the dead. Thus they provoked him to anger with their doings; And the plague brake in upon them. Then stood up Phinehas, and executed judgment; And so the plague was stayed. And that was reckoned unto him for righteousness, Unto all generations for evermore." — Psalms 106:28-31 (ASV)

The licentious character of the cult of Baal-peor in Numbers 25 is expressed in the word “joined,” or, better, yoked. The Septuagint and Vulgate read, “were initiated,” that is, by prostitution.