This is an interesting read.
Excerpts:
We know that God does not equate sexual intercourse with marriage based on the fact that the Old Testament often distinguishes a wife from a concubine.
Also, 1 Corinthians 7:2 indicates that sex before marriage is immorality. If sexual intercourse causes a couple to become married, it could not be considered immoral, as the couple would be considered married the moment they engaged in sexual intercourse.
Some point to Genesis 24 and the story of Isaac and Rebekah as an example of a couple being married solely by sexual intercourse, without any type of ceremony. But the details that lead up the marriage reveal that a formal process was followed.
1 Corinthians 7:34 There is difference also between a wife and a virgin. The unmarried woman careth for the things of the Lord, that she may be holy both in body and in spirit: but she that is married careth for the things of the world, how she may please her husband.
Virgin = unmarried, not haven made the sexual union
wife = married, having made the sexual union