Leviticus 15:18 - Ellicott's Commentary On The Whole Bible

The woman also with whom man shall lie. — Better, And if a man lie with a woman, that is, even when what is specified in Leviticus 15:16 takes place in intercourse between man and woman lawfully married, it pollutes both the husband and the wife. They have accordingly both to immerse their whole bodies, and remain unclean till sundown, and were debarred from the privileges of the sanctuary during that day. Hence abstinence from conjugal intercourse was regarded as a necessary preparation for the performance of sacred duties. He who had approached his wife could not draw nigh to God (Exodus 19:15), and was not allowed to partake of sacred meals. (Comp. 1 Samuel 21:5-6.) The law of pollution was not designed to put a check upon marriage, since matrimony is a Divine institution (Genesis 1:27-28; Genesis 2:21-25), but it is intended to prevent husband and wife from making an immoderate use of their conjugal life, and thus to preserve them in health and vigour by prescribing such constant purifications after it. This is probably the reason why other nations of antiquity enacted similar laws. Thus the Hindoos and the Babylonians bathed after conjugal intercourse. The Egyptian priests abstained from it when they had to perform sacred duties, and the laity were not allowed to enter the precincts of the Temple unless they submitted to ablutions. Mahommed, for the same reason, enjoins lustrations upon all the faithful before reciting their prayers.