If a man lies with a woman and there is an emission of semen, both must bathe with water, and they will remain unclean until evening.
We have here the law concerning the ceremonial uncleanness that was contracted by running issues in men. It is called in the margin (Lev 15:2) the running of the reins: a very grievous and loathsome disease, which was, usually the effect and consequent of wantonness and uncleanness, and a dissolute course of life, filling men's bones with the sins of their youth, and leaving them...
Commenting on Leviticus 15:1-18
The woman also with whom man shall lie with seed of copulation,.... It seems to respect any congress of a man and woman, whether in fornication or adultery, or lawful marriage, and particularly the latter; for though marriage is honourable and holy, and carnal copulation in itself lawful, yet such is the sinfulness of nature, that as no act is performed without pollution, so neither...
They shall both bathe themselves - What a wonderful tendency had these ordinances to prevent all excesses! The pains which such persons must take, the separations which they must observe, and the privations which, in consequence, they must be exposed to in the way of commerce, traffic, etc., would prevent them from making an unlawful use of lawful things.