Practice of a married woman having extra marital relationship

One word substitute is Adultery.

Polygamy: the practice or custom of having more than one wife or husband at the same time.
Puberty: the period during which adolescents reach sexual maturity and become capable of reproduction.
Lechery: excessive or offensive sexual desire; lustfulness.
Adultery: voluntary sexual intercourse between a married person and a person who is not their spouse.