Adam Clarke on Numbers 25:3

Israel joined himself unto Baal-peor - The same as the Priapus of the Romans, and worshipped with the same obscene rites as we have frequently had occasion to remark.

The joining to Baal-peor, mentioned here, was probably what St. Paul had in view when he said, 2Cor 6:14 : Be ye not unequally yoked together with unbelievers. And this joining, though done even in a matrimonial way, was nevertheless fornication, (see Rev 2:14), as no marriage between an Israelite and a Midianite could be legitimate, according to the law of God. See the propositions at the close of the preceding chapter Num 24:25 (note).