Why does the Torah prohibit entering into a covenant with the Cana'anim and eating from their sacrifices?
Rashi: The Torah views eating from their sacrifices akin to participating in their idol-worship, because one thing leads to another, and the next step is taking his daughter as a wife for one's son.
Ramban: Straying after the gods of the Cana'anim, eating from their sacrifices 1 and taking their daughters for our sons are all prohibitions that stem from making a covenant with them.
Seforno: Because entering into a covenant with them sometimes leads to initially to participating in their festivities and from there to joining them in their idolatrous practices, and sometimes to licenteousness. 2