From The Aryeh Kaplan Anthology II pg. 326

The Torah forbids sexual contact between man and a woman in the status of Niddah. "You shall not come close to uncovering the nakedness of a a woman who is 'unclean' as a Niddah." (Leviticus 18:19).

The seriousness of the sin is told in the Torah. "If a man lies with a woman who is a Niddah, and uncovers her nakedness... both of them shall be cut off from their people." (Leviticus 20:18).

"They shall be cut off" refers to Korais, the penalty of incest between brother and sister, violating Yom Kippur, or eating bread on Passover. The expression does not refer to mutilation or excommunication, rather a spiritual penalty of being cut off from one's spiritual source. The individual loses his ability to feel and appreciate the spiritual and G-dly, and thus becomes "cut off" from the most important elements of life as a Jew.

to be continued