At this point, we would do well to stop and ask ourselves why the state has anything to say about marriage at all. Why does the state outlaw polygamy? Why does it regulate divorce? Why does it give any favorable or unfavorable treatment to married couples?
Maybe marriage is church business after all and the state should just butt out. Many actually recommended repealing all the marriage laws and leaving it to churches to do as they will.
Well, there is a reasoning behind these things. And it can be found in the Bible. If God established and ordered marriage, he had a reason for doing so. And that reason, irrespective of religion, may be why the state has an interest. We can start (as most biblical conversations on marriage do) in Genesis 1.