Adultery: In Genesis 2:24, the Bible tells us of the origin of the institution called marriage, the union of a man and a woman in holy matrimony. The act of having sexual affairs outside of marriage is simply called ADULTERY and whosoever indulges in such act is known as an ADULTERER/ADULTERESS.
God is strongly against it because in Exodus 20:14, He commanded His people not to commit adultery. When a man finds a woman worthy to be his wife, after marrying her, he is not to look at or covet another woman. The other woman according to the Bible in the book of Proverbs is referred to as the 'strange woman'.