Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)By the Chofetz Chaim

This book lists the Torah mitzvos that can be observed today

Second Section – the Negative Commandments

124. It is a negative commandment “do not have relations with a married woman”

As it states in Shemos (Exodus) 20:13, “do not commit adultery” and it states in Vayikra (Leviticus) 18:20, “do not lie with your friend’s wife.” Both the man and woman receive the death penalty of strangulation. If she is a daughter of a Cohain, she receives death by burning and he receives death by strangulation. If she is a virgin woman betrothed to another man and she still lives in her father’s house, both of the partners receive death by stoning. All of these only apply if there are witnesses and warning. If the act was done with intention but without warning, the man and woman receive spiritual excommunication. If done by error, they need to bring a sin offering.

This applies in all places and at all times.