There are at least 5 purposes for marriage that I can think of-
1. Marriage is for relational fulfillment and intimacy
The first marriage addresses this basic purpose for marriage in the latter part of Genesis 2:18-"Then the Lord God said, 'It is not good that the man should be alone; I will make him a helper fit for him.' " Literally this means "a helpmate suitable for him". Husbands and wives are to fit. They complement each other. They relate to one another. Shari fits me and is who I need to be complete relationally. According to this foundational verse for marriage, there is a certain way God designs wives so they bring completion to their husbands. Two parts that fit enjoy a dynamic, ongoing relationship. More basic to this, there is relational fulfillment in being married to my wife. Things are always interesting. I have never come to the end of figuring her out, and I'm guessing she'd say the same about me! This keeps things interesting. In addition to relational fulfillment is the unique intimacy reserved for marriage-
Proverbs 5:15-19 Drink water from your own cistern, flowing water from your own well.  Should your springs be scattered abroad,streams of water in the streets?  Let them be for yourself alone, and not for strangers with you.  Let your fountain be blessed,and rejoice in the wife of your youth,  a lovely deer, a graceful doe. Let her breasts fill you at all times with delight; be intoxicated always in her love.
Sexual intimacy is special and powerful because it is exclusive. Something is not intimate if it is widespread or shared abroad. Misuse is devastating. Right use is uniquely blessed.
2. Marriage is for the bearing of children
There are certainly exceptions, however the general norm for marriage is the blessing of bearing children. Only 28 verses in to the book of Genesis we read-
Genesis 1:28 And God blessed them. And God said to them, "Be fruitful and multiply and fill the earth and subdue it and have dominion over the fish of the sea and over the birds of the heavens and over every living thing that moves on the earth."
The most effective discipleship occurs in the family. Raising godly children strengthens the Church and emboldens the witness of Christ in the world. More personally, I have found my children to be tremendous aids in my sanctification and promising hunting partners.
3. Marriage is for the elimination of loneliness
Straight up, Moses wrote (Genesis 2:18)- "It is not good that the man should be alone; I will make him a helper fit for him." Don't get me wrong, it's better to be unmarried than to marry the wrong person, however in general, being married is very good, especially because it is a cure for loneliness. Loneliness refers to an emotional state in which a person experiences a powerful feeling of emptiness and isolation. This is not to say that every unmarried person feels lonely, but rather marriage works wonderfully, by design, to prevent and eliminate such a sense. Loneliness is a heavy burden. Even with all that was good and pure in the world before the Fall, Adam would have been incomplete and thus somehow unfulfilled had God not provided Eve.
4. Marraige is for the prevention of immorality
Scripture speaks pretty bluntly about one area marriage helps us avoid sin in:
1 Cor. 7:2 But because of the temptation to sexual immorality, each man should have his own wife and each woman her own husband.
I would further point out the general helpfulness having a godly helpmate provides. My wife has helped keep me from anger, from making poor financial decisions, and in numerous other protective ways. She is my chief accountability partner as I am for her. Shari helps my walk with Christ like no other.
5. Marriage is for societal order and health
There are three divine institutions- The Church, Government, and Family. Humanly speaking, the success and effective function of the first two depend heavily on the integrity of the last. If you are married, you are a family. Children do not make a family- a husband and wife do. Children may be added to an existing family, however, the unique one-flesh union called marriage is also properly understood as a family. Far too many families are child-centered. They should be Christ-centered with Dad and Mom as the leaders in this focus, not Dad and Mom catering to the desires of the children...but I digress, that's a subject for a different post. God’s plan for marriage and the family has been the building block of solid societies for millennia. Conversely, societies suffer when they embrace alternative forms of the family, but again, that's a subject for a different post.
More thoughts tomorrow. The warm fuzzies are abounding.