C. F. Keil and Franz Delitzsch on Leviticus 15:18

Lev 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; 1Sa 21:5-6; 2Sa 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.)