(Proverbs 22:14 KJV) The mouth of strange women is a deep pit: he that is abhorred of the LORD shall fall therein.
The definition for “strange women” (adulteress NASB) (2114a) is basically “stranger.” In the Hebrew there is one subset in the definition in which it means “prostitute” or “harlot.” I can only assume that is the case here. The NASB has several different words translated “adulteress.” The KJV only uses the English word three times, never of this one.
In this case the idea is that two people are violating the marriage covenant. In the Bible the act of sexual intercourse is often called “to know.” It is the ultimate knowing. It is yada, a word that is also used to describe knowing God. Sex in marriage has that high a place in God’s plan. To get involved outside marriage is a denial of that covenant and is second only to idolatry which you can think of as adultery toward God.
So? There are commitments we make in life. We make them to God. We make them to our spouse. We break that covenant at the risk of eternal hell. Morality matters. Faithfulness is a basic in the life of righteousness.