RE: I thought adultry was also having sex before marriage?
Having sex with someone other than your spouse after marriage is adultery. 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.
Both are condemned by Scripture.
Matthew 15:19:
For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies ... (KJV)
Flee from it:
1 Corinthians 6:18
Proverbs 6:32