Leviticus 18:8

Father's wife] This is not the same as 'mother' in the previous v. so that polygamy is here presupposed. It was common, perhaps universal, in the East at the time of Moses. The Mosaic Law did not seek all at once to abolish polygamy, which might have been the occasion of great hardship in the circumstances. But it certainly discouraged it, and by regulating and restraining it prepared the way for its gradual extinction: cp. the remarks on slavery among the Hebrews at Exodus 21.