John Wesley on Numbers 31:7

All the males - Namely all who lived in those parts, for colonies of them, were sent forth to remoter places, which therefore had no hand either in their former sin, or in this present ruling. And herein they did according to God's own order concerning such people, Deu 20:13, only their fault was, that they did not consider the special reason which they had to involve the women in the destruction.