More commonly it's called premarital sex. And you don't need to be married to commit adultery, boyfriends and girlfriends cheat on (commit adultery) all the time.. and they're not married.
Lets stick with scripture to simplify things okay?
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Hebrews 13:4[/h][h=3][/h][h=3][/h] Let
marriage be held in honor among all, and let the
marriage bed be undefiled, for God will judge the sexually immoral and
adulterous.
Adultery is committed by those who are
married.
AND...........
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Matthew 5:32[/h][h=3][/h][h=3][/h] But I say to you that everyone who divorces his wife, except on the ground of
sexual immorality, makes her commit
adultery, and whoever marries a
divorced woman commits adultery.
Under certain circumstances it is adultery.
One or more has been married and has divorced under unbiblical terms
"This collection of Scriptures is provided as an aid for those who wish to study what the Bible says about adultery and fornication.
Adultery is the act of sexual intercourse between a married man and someone other than his wife, or sexual intercourse between a married woman and someone other than her husband. Adultery violates the bond of the
marriage union. Fornication is a more general term usually referring to any kind of sexual misconduct or sexual impurity outside of the bounds of marriage."
Matthew 15:19
For out of the heart proceed evil thoughts, murders,
adulteries, fornications, thefts, false witness, blasphemies ...
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Revelation 22:15[/h][h=3][/h][h=3][/h] Outside are the dogs and sorcerers and the sexually immoral and murderers and idolaters
. Here is another excerpt from gotquestions.org
In the New Testament, "fornication" comes from the Greek word
porneia, which includes adultery and incest.
Porneia comes from another Greek word that also includes indulging in any kind of unlawful lust, which would include homosexuality. The use of the word in the gospels and the epistles is always in reference to sexual sin, whereas "fornication" in the book of Revelation always refers to idolatry. The Lord Jesus condemns two of the churches of Asia Minor for dabbling in the fornication of idolatry (
Revelation 2:14,
20), and He also refers to the “great harlot” of the end times, which is the idolatrous false religion “with whom the kings of the earth committed fornication, and the inhabitants of the earth were made drunk with the wine of her fornication” (
Revelation 17:1-2, NKJV).
Adultery, on the other hand, always refers to the
sexual sin of married people with someone other than their spouse, and the word is used in the Old Testament both literally and figuratively.
The Hebrew word translated “adultery” means literally “breaking wedlock.” 
