Understanding the origins of marriage is the key to understanding marriage itself. We see this in Jesus' interactions with the Pharisees (Matthew 19:1-10; Mark 10:1-12), who once asked Him for His opinion on the legitimacy of divorce. His response was to take them straight back to the beginning, citing Genesis 1:27 and 2:24. Here, He points out that God did not design marriage to end. Rather, it is when a man leaves his mother and father (i.e. the previous married couple) and is joined to his wife, and the two become one flesh.
That phrase "one flesh" is really quite deep. The human body is quite capable of surviving on its own. We all have a brain. We all have a heart. We all have lungs. Every function our bodies need to perform, we have in ourselves. But there is one function we cannot perform alone: Reproduction. What one spouse has, the other spouse lacks. The wife has a womb, which her husband lacks. The husband produces sperm, which his wife cannot do. Men and women are fundamentally different, and require each other to complete the human function.
We even see this in 1 Corinthians 6:16, where Paul says that to sleep with a prostitute makes a man one flesh with her. This shows us just how sacred sex is. It is designed exclusively for marriage, which is the permanent and exclusive union of one man and one woman. Notice, this is not a temporary arrangement. It is not moral to marry, then divorce. It is not moral to skip marriage and sleep with a prostitute. It is not moral to jump the gun and sleep with someone before you marry them.
Furthermore, gender is important. The Bible actually describes homosexuality as unnatural, and almost as a natural consequence and punishment for rejecting God.
Even polygamy takes a few hits when we understand Scripture. It is a grey area; we can debate for hours about what to do if a man with multiple wives becomes a Christian. But Scripture undoubtedly portrays monogamy as the superior model. Anything more even precludes one from church leadership (1 Timothy 3:2, 12).
Adam and Eve, having the first marriage, are the picture of what it should be. One man and one woman for life. But of course, there is a third party involved: The God who started it all. Without Him, marriage is meaningless. You only need to look at the sorry state our culture is in to realise this. But when both husband and wife submit to God, marriage is a truly beautiful thing. Let us therefore seek to honor God with our marriage practices.