Sexual intercourse during a woman’s menstrual cycle was ruled out in Leviticus 15:19, Leviticus 15:24. Leviticus 15:1–33, however, dealt with various kinds of unintentional contact with things that are unclean. If sexual intercourse was due to an accident in timing, then a period of uncleanness and the subsequent ritual purity were required. Conversely, Leviticus 20:1–27 is set within the context of deliberate sin. Such intentional sin is contrary to the will of God and receives his judgement.
18 If a man lies with a woman during her menstrual period and uncovers her nakedness, he has made naked her fountain, and she has uncovered the fountain of her blood. Both of them shall be cut off from among their people.