See the context of this verse, Ezekiel 18:3-9
ESV - 6 If he does not eat upon the mountains or lift up his eyes to the idols of the house of Israel, does not defile his neighbor's wife or approach a woman in her time of menstrual impurity.
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Wiersbe said that it was one of 8 negative offenses/sins that would “incur ritual uncleanness” and that this man avoids. Dr. Wiersbe also says Menstruation means uncleanness (Ezek. 36:17) and most importantly see Lev. 15:19-30. In ancient Jewish law, a woman in her menstrual cycles was considered ceremonially unclean and not able to do worship (Lev 15:19-30). A man who had sex with her also was ceremonially unclean and unable to do worship. While the law was only partly hygienic, it was mainly based on God’s wish that his people be pure in worship and relationships.