From The Aryeh Kaplan Anthology II pg. 326

The only way for a person to re-attach himself to his spiritual source is to sincerely repent before G-d with resolve never to repeat the act.

Ezekiel equates intercourse in Niddah with adultery "If a man is righteous and would accomplish justice and charity,... he will not defile his neighbor's wife nor approach a woman who is a Niddah." (Ezekiel 18:5,6)

Removal of status of Niddah by immersion in a Mikvah is known primarily from the Oral Torah, transmitted by G-d to the Jewish people at Sinai along with the written Torah. It is alluded to in the Torah: "She shall count seven days and then she shall be purified." (Leviticus 15:28). After coundint seven clean days, a woman must undergo a normal process of ritual purification. Universal means of purification is via immersion in Mikvah. This universal means of purification applies to a Niddah. even though the prohibition against sex for a Niddah is not directly related to ritual impurity (Tumah), the means of changing this status is likened to a purification process.

to be continued