“If a man commits adultery with the wife of his neighbor, both the adulterer and the adulteress shall surely be put to death. If a man lies with his father’s wife, he has uncovered his father’s nakedness; both of them shall surely be put to death; their blood is upon them. If a man lies with his daughter-in-law, both of them shall surely be put to death; they have committed perversion; their blood is upon them.”
The punishment for adultery is the death of both parties because adultery terminates the prior marriage (i.e., the one-flesh union) and begins another marriage (even if an imperfect one). Both parties are executed because they are, in truth, one party.