Adultery is considered 'sin' in the true messages of all three Abrahamic religion (Israelite, Christianity and Islam). However, later some of the revealed messages were corrupted to make them attractive and to recruit more sheep. St. Paul took the extreme position to combat adultery. He forbided clergy to mary, something which has backfired long time ago. Curently, according to US Churches - over 40% of Nuns have been sexually abused in their life time.
Jewish Talmud took a step further - it defined adultery as a sexual relationship between a Jewish man and wife of another Jewish man. If both are not married or only man is married or the act of sex is between Jewish man and a non-Jewish women (married or unmarried) - it's not adultery.
Islam, being uncivilized, according to western standards - still hang on to the 1400-year-old command - sex between a male and female (Muslims or non-Muslim) out of marriage, is a major sin, punishable with stonning in a truely Islamic State.