With the definition of marriage up for grabs in our culture, it's incumbent on Christians to come up with our own understanding of what it's about and its purpose.

Mike Leake suggests this as a definition:

Marriage is a binding covenant created by God between one man and one woman for our holiness, for our joy, as a picture of the gospel to spread the glory of God.

If I pick this definition apart I can come up with seven individual statements (and these are used as teaching points for the first session).
1. A Binding Covenant. Covenants are a big deal to God. Breaking covenants is a big deal to God. To see how big of a deal covenants are consider Genesis 15. The Lord walks through a host of animals that are ripped asunder and essentially says, “If I break my covenant let what is done to these animals be done to me”. Covenants are a big deal.

2. Created by God. If humans created marriage then we could make the rules. But marriage is a binding covenant that is created by God, as such He makes the rules. God created your marriage, so away with this silly talk of having “married the wrong person”.

3. Between one man and one woman. The two shall become one. This means breaking away from parents, past relationships, future relationships, and any other lovers. This also goes against any arguments for homosexuality rightly being called marriage.

4. For our holiness. Marriage is one of the means that God has ordained to sanctify us. God is not satisfied with us merely having a “good” marriage. God wants to use our marriage to conform us more and more into the image of Christ. God has a rescue plan for your marriage. His goal is not simply to rescue your marriage. His goal is to use your marriage to rescue you.

5. For our joy. Our joy increases when we, in holiness, fight for the joy of another. Marriage can be extremely joyful. Just read Song of Solomon. Furthermore, if marriage increases holiness it will also increase our joy in God.

6. As a picture of the gospel. Your marriage reflects Christ and His church. It was created by God to be a visible picture for everyone to see the love between Christ and His Bride.

7. To spread the glory of God. The purpose of God for humanity is to enjoy His grace and extend His glory. Marriage is no different. He uses marriages to rip out of our heart sin and unbelief. He uses marriage to further our joy. But he also uses marriage to create children, and to raise and nurture children in godly homes.