MENSTRUATION
The Law prohibited sexual contact with women during their menstruation (Lev. 15:19-24 regulates periodic menstruation, while Lev. 15:25-28 deals with the issue of irregular or pathological menorrhagia).
In the OT it was considered an impurity, in the same way that every firstborn of man who opened the womb had to be redeemed by sacrifice (Ex. 13:13; 34:20).
The reason is evidently that man is considered a sinner and impure, and that everything that has to do with human nature itself and the transmission of this sinful life is considered impure, and requires sacrifice for its purification.