The Hebrew word translated as defile yourself can be more literally translated as to become ceremonially unclean.
In the Old Testament, sexual intercourse with a neighbor’s wife or any married woman was adultery. Breaking this law was punishable by death (Deuteronomy 22:22). Like incest, adultery threatened the integrity and security of the home and family. It violated the marriage covenant and divided the adulterer’s affection between his spouse and his mistress.