Marriage is not just a social institution. It is a gift from God to us to illustrate our covenant relationship with Him, through Christ.
"'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one.' This is a great mystery, and I take it to mean Christ and the church"
In this passage of scripture addressing husbands and wives, Paul takes a detour to give us a beautiful picture of the relationship of Christ with the Church.
In this section, Paul says Christ gave himself up for the church. At the last supper, he declared his intent to lay down his life to form a new covenant relationship with all who would accept it. He offered the bread and wine saying, "this is my body," "this is my blood," and told his followers that this was the beginning of a new covenant. Covenant is an interesting concept that we often mistake for "contract" today. Covenants with God offer a much stronger bond in which He commits to uphold that bond regardless of a person's ability to uphold their part of the relationship. Our marriage relationships ought to reflect this same commitment level--for better or worse, richer or poorer, in sickness and health, until death separates. This kind of commitment on our part is only possible because we have this covenant relationship with God.
Christ not only gave himself up for the church, he did this to cleanse the church and make her radiant. He is working in each of our lives to prepare us for the day when we will be united with him in perfect unity. On that day, Revelation 19:8 that on that day the bride of Christ will be given fine linen to wear representing the righteousness of the saints--which comes from Christ.
"Blessed are those who are invited to the wedding supper of the lamb."
If we have accepted the covenant relationship with Christ, we are part of the church. We have been invited to the wedding feast, not as guests, but as the very bride of Christ.