I'm with CatHerder.
1 Corinthians 6:9 - "Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind,"
Fornication - "sexual intercourse between people not married to each other."
If sex is marriage then this word has no meaning; it is impossible.
I agree that marriage is not run by the state, and therefore if you and your significant other decide to get your relatives and go have a priest marry you before God in some bush, without inviting Uncle Sam, then that's fine. But this is a very different thing then fornicating with your fiance and then saying, "We are married in God's eyes" which I have heard so often said. I understand that people here are trying to say that sex is a very serious bonding, which is true, but not all serious bondings are marriage. Also consider that if sex is marriage, then there is also no such thing as divorce (sex cannot be undone) and all sexual activity done thereafter is adultery. But the bible talks about two cases where divorce is tolerable,
"And I say to you: whoever divorces his wife, except for sexual immorality, and marries another, commits adultery." - Matthew 19:9
"But if the unbelieving partner separates, let it be so. In such cases the brother or sister is not enslaved. God has called you to peace." - 1 Corinthians 7:15