Are you asking because you care what God thinks are you just asking for the sake of arguing your viewpoint? I am grieved when people do things that are against God's way of doing things, why? because I know that doing things His way always produces good. (and by the way God's idea of good is not man's idea of good) Man thinks that good means you will never have problems, not so. Just because you marry doesn't mean you will not encounter all the same things if you are common law married.
The Lord tells us in His word that we are to obey the law of the land if it lines up with His word (the Bible). The law of the land tells us that we have to have a piece of paper in order to be married. Therefore, since marriage is an institute God created then God is all for making marriage legal, ie a piece of paper. If God says marriage is good then it is good. If God says to obey the laws of the land then we are to sign a piece of paper in order to be married. In this country if you do not sign a piece of paper then you are not considered married. Living together for a long period of time, having children together does not make you married according to the laws of our land.
That small piece of paper cements your committment and makes it legal. It is a whole lot easier to walk away from a contract or committment when you do not have any binding legal paperwork tying you down. Now for my own personal opinion--if you care to know.... People who do not have legal paperwork saying they are married are afraid of something and frankly cannot commit all of their hearts to that one person. They can have sex with them, have children (which by the way costs much more than a $50.00 marriage certificate), and live as a married person, but they can't seem to seal the deal with a marriage certificate....what's wrong with this picture? And why are these people above the law anyway, if the law says in order to be married you have to sign a piece of paper and pay a few bucks to the government, so what. The price of one's love has no price tag or so it shouldn't.
We can agree to disagree. My original point was to say that God is in favor of marriage and He is not in favor of living together.