This question comes from Twaambo:
"Where in the bible does it say that for a marriage to be ordained by God a couple must be blessed by a priest and also must exchange vows? Isn't it in most cases we read ... 'and David knew his wife', or 'Abraham knew Sarah' etc. and as long as they consummated then in God's eyes they were married?"
The short answer is, it doesn't.
Nowhere in the Bible does it seem to indicate that God commands or requires some "official" ceremony for a couple to be married (but the Bible does clearly define what a couple is made up of - one man and one woman, but that is a different discussion). Two becoming one flesh is the designated sign of a couple being "married" in God's sight (Gen 2:24; 1 Cr 6:16; Eph 5:31) and Jesus references God's original intent in Genesis in his teaching to the Pharisees as to when a couple becomes one. The one should never become two again by human will (Mat 19:5-6).
However, having said all that, there is another truth in play here, and that is the principle our humble submission as Christians to the governing authorities (Rom 13:1-7) , on account of our faith in God and his sovereign reign in and over the world.. If a Christian (someone under the new covenant of Christ) says that they don't want to submit to the laws of the land and go through the proper legal and civil processes in order to be married (Rom 13:2) and relies on the argument that in God's sight marriage truly begins at sexual consummation, he or she is being nearsighted and could possibly have their heart ultimately set on selfish desires, which opposes the heart of Christian marriage anyway (Eph 5).
While it is true that God's original intent seems to be that "marriage" begins as "two become one flesh" (sexual consummation), as Christians we must also humbly and willingly submit to the governing authorities whom we find ourselves under, because our submission to them is the same as our submission toward God (Rom 13:2).