Leviticus 15:18

Sexual connection. "If a man lie with a woman with the emission of seed, both shall be unclean till the evening, and bathe themselves in water." Consequently it was not the concubitus as such which defiled, as many erroneously suppose, but the emission of seed in the coitus. This explains the law and custom, of abstaining from conjugal intercourse during the preparation for acts of divine worship, or the performance of the same (Exo 19:5; Sa1 21:5-6; Sa2 11:4), in which many other nations resembled the Israelites. (For proofs see Leyrer's article in Herzog's Cyclopaedia, and Knobel in loco, though the latter is wrong in supposing that conjugal intercourse itself defiled.)

In this chapter we have laws concerning other ceremonial uncleannesses contracted either by bodily disease like that of the leper, or some natural incidents, and this either, I. In men (v. 1-18). Or, II. In women (Lev 15:19-33). We need not be at all curious in explaining these antiquated laws, it is enough if we observe the general intention; but we have need to be very cautious lest sin take occasion by the commandment to become more exceedingly sinful; and exceedingly sinful it is when lust is kindled by sparks of fire from God's altar. The case is bad with the soul when it is putrefied by that which should purify it.

15:18 If the seminal emission involved sexual intercourse, both the man and the woman became unclean and had to wash and remain unclean until the next evening. Although it involved a bodily emission, sexual intercourse was not an infection or abnormality, and it required no sacrifices.