I had this question here before, Here is what I put together for someone who asked the same thing:
Definitions are very important…
adultery (Latin: adulterium)
Carnal intercourse of a married person and another who is not the wife or husband. It is a diriment impediment to marriage between two who, during the time of a legitimate marriage, commit the crime pledging themselves to marriage later; or, who commit it during the time of a legitimate marriage and one or the other brings about the death of one of the married parties.
fornication:
The free act of carnal intercourse between persons who are not married but who are free to marry. Fornication is always gravely wrong because it deprives sexual intercourse of the order which the good of the human race demands. It is a substantial inversion of an essential order.
Above definitions from:
www.catholic-forum.com/saints/indexncd.htm
Matt.15 6-21
Mark.7 18-23
Gal.5 13-26
Eph.5 1-17
Col.3 1-14
1 Tim 1 8-10
1Cor 6 9-20
I used these verses with someone who wanted to a) Live w/ someone before marriage, and b) Have pre-marital sex after becoming Engaged. This person was open to the Truth of the Scriptures, and shortly re-gained her and her betrothed’s purity and maintained it for over 1 1/2 years. They just got married. Praise God.
I also believe Dr. Colleen Kelley Mast from The Dr. is in Radio Show has statistics on her website as regards to this issue and how it is NO help before marriage from a practical standpoint (i.e. compatibility, etc.).
I will pray for your intentions.
Bob