When a man and his wife had intercourse in which there was an emission of semen, they were required to bathe and were unclean until evening. (Le 15:16-18) If inadvertently a wife’s flow began during intercourse, then the husband was unclean seven days, the same as his wife. (Le 15:24) If they deliberately showed contempt for God’s law and had sexual relations while she was menstruating, the penalty of death was imposed on the male and the female. (Le 20:18) For the above reasons, when ceremonial cleanness was required, as, for example, when men were sanctified for a military expedition, they were obliged to refrain from having intercourse with their wives.​—1Sa 21:4, 5; 2Sa 11:8-11.

Why did the Mosaic Law say that sexual intercourse and childbirth made a person “unclean”?

The question arises: Why were such normal, proper things as menstruation, sexual intercourse between married persons, and childbirth viewed in the Law as making one “unclean”? For one thing, it raised the most intimate relations of marriage to the level of sanctity, teaching both mates self-control, a high regard for the reproductive organs, and respect for the sacredness of life and blood. The hygienic benefits that accrued from scrupulous observance of these regulations have also been commented on. But there is yet another aspect of the matter.

In the beginning God created the sex impulses and generative powers in the first man and woman and commanded them to cohabit and bring forth children. It was therefore no sin for the perfect pair to have sexual intercourse. However, when Adam and Eve disobeyed God, not in the matter of sex relations, but in eating the forbidden fruit, drastic changes took place. Suddenly their guilty sin-stricken consciences made them aware of their nakedness, and they immediately covered their genital organs from God’s sight. (Ge 3:7, 10, 11) From then on, men could not carry out the procreative mandate in perfection, but, instead, the hereditary blemish of sin and the penalty of death would be transmitted from the parents to children. Even the most upright and God-fearing parents produce sin-infected children.​—Ps 51:5.

The Law’s requirements pertaining to the functions of the reproductive organs taught men and women self-discipline, restraint of passions, and respect for God’s means of propagation. The Law’s regulations forcefully reminded creatures of their sinful state; these were not merely health measures to ensure cleanliness or prophylactic safeguards against the spread of diseases. As a reminder of man’s inherited sinfulness, it was fitting that both the male and the female with genital discharges due to normal functions of their bodies observe a period of uncleanness.