John Gill Commentary Leviticus 18:20

"And thou shalt not lie carnally with thy neighbor`s wife, to defile thyself with her." — Leviticus 18:20 (ASV)

Moreover, you shall not lie carnally with your neighbour's wife Which is adultery, and a breach of the seventh command, (Exodus 20:14) : to defile yourself with her ;

not only adultery is a defiling a man's wife, as it is sometimes called, but the adulterer defiles himself: all sin is of a defiling nature, but especially this, which defiles a man both in soul and body, and brings a blot and stain upon his character, which shall not be wiped off, (Proverbs 6:32Proverbs 6:33) .