Fornication historically: 'Fornication' in Paul's writings and in Jesus time
perhaps refers less to a sexual act and more to having children out of wedlock. It might only refer to the second thing. Modern Catholicism seems not to view it that way seeing the sex itself as sinful. That is why the dictionary defines fornication as sex, but what fornication meant in the past may have been different. Its hard to tell.
Using only the Bible and trying to figure out what by fornication is meant, one of the verses that strikes me as particularly ambiguous is I Corinthians 7:1-2 "Now concerning the things whereof ye wrote unto me:
It is good for a man not to touch a woman. Nevertheless,
to avoid fornication, let every man have his own wife, and let every woman have her own husband." The verse itself doesn't describe what fornication is. You're just supposed to know already. Moving backwards in time to the laws there is no law against sex that I can see, but there are laws about having sex with a married person or with family members.
There is an interesting article about the word 'Fornication' in the Jewish encyclopedia.
http://www.jewishencyclopedia.com/articles/6238-fornication