In layman's terms, fornication is often used as an allegory for idolatry. In the same way people are unfaithful to spouses by fornicating in the usual sense of the word, idolatry is "cheating on" God.
More at http://www.thewholetruthnow.com/2PORNEIA.htm and http://www.sabbathcovenant.com/doctrine/fornication_defined.htm
From the second link above:
In Revelation, we again see a clear link between the act of
fornication and idolatry:
14 But I have a few things against thee, because thou hast there them that hold the doctrine of Balaam, who taught Balac to cast a
stumblingblock before the children of Israel, to eat things sacrificed
unto idols, and to commit fornication....
20 Notwithstanding I have a few things against thee, because thou sufferest that woman Jezebel, which calleth herself a prophetess,
to teach and to seduce my servants to commit fornication, and to eat
things sacrificed unto idols.
John is clearly speaking out against the practice of committing the
sexual act of idolatry and then eating meat sacrificed to idols in
that act of worship.
If you do a word study on Fornication, you will see that it is
condemned in context to Idolatry EVERY time. 100% of the references
to fornication are in context to idolatry.
similarly, several times in Scripture, idolatry is described as "whoring after other gods" such as in
Judges 2:17 KJV
And yet they would not hearken unto their judges, but they went a
whoring after other gods, and bowed themselves unto them: they turned
quickly out of the way which their fathers walked in, obeying the
commandments of the LORD; but they did not so.