According to Longman dictionary, it defines the marriage as the union of a man and woman by a ceremony in law, it further state that marriage is the state of being so united and it last all one's life. With these definition it is clear that marriage is an ordained and oldest institution of God.
Other creatures created before Adam by God was created in double, that is female and male, but as that of Adam, he was created alone. This prompted Almighty God who decided to make a helpmate for Adam. Since God is an action God, He immediately swung into action in Genesis 2:2-24. With this action taken by God, that is joining Adam and Eve together, marriage institution took effect from that day till now at that point.
Marriage is a divine plan of God, joining of man and woman to become one flesh, this leads to husband and wife. In the beginning, after the creation of all things, the last thing that was created was man, I. e. Adam.
In the garden of Eden, God noticed that for Adam to be alone is a problem, because Adam is different among other creatures in the garden of Eden, different in language, reaction, thinking, eating, working, movement and in interaction. He interacted with God, the animal, bird in the garden, but no other person like Adam.
The Bible said in genesis 2:18, 21 and I quote;
And the LORD God said, It is not good that the man should be alone; I will make him an help meet for him. And out of the ground the LORD God formed every beast of the field, and every fowl of the air; and brought them unto Adam to see what he would call them: and whatsoever Adam called every living creature, that was the name thereof. And Adam gave names to all cattle, and to the fowl of the air, and to every beast of the field; but for Adam there was not found an help meet for him. And the LORD God caused a deep sleep to fall upon Adam, and he slept: and he took one of his ribs, and closed up the flesh instead thereof. Genesis
Marriage is a companion of suitable man and woman coming together under one umbrella to form a home, and do everything in common without shame, God is the founder of marriage.