Now, fornication has come to mean premarital sex almost exclusively in modern English, but this is not its meaning in the Greek, nor the meaning of uncleanness or nakedness of a thing in the Hebrew. Now, since every act of extramarital sex by a husband or wife with a person of the opposite sex is adultery, even though it might be also perversion or it might be incest, it is still adultery. To use a word other than adultery means something else than merely sexual offenses. It refers to this fact of lasciviousness and rebelliousness, and unbelief.
Audio lectures - The Law of Divorce (n.d.)