Ephesians 5:31-33 (NIV) says "For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh.” This is a profound mystery — but I am talking about Christ and the church. However, each one of you also must love his wife as he loves himself, and the wife must respect her husband."
In trying to understand more about what this passage says, I looked at The Message translation. It says "And this is why a man leaves father and mother and cherishes his wife. No longer two, they become "one flesh." This is a huge mystery, and I don't pretend to understand it all. What is clearest to me is the way Christ treats the church. And this provides a good picture of how each husband is to treat his wife, loving himself in loving her, and how each wife is to honor her husband."
This is a wonderful depiction of God's relationship with us. Unlike parental love which is held togther by blood ties, spousal love is special. A man meets a woman; by falling in love, he chooses her. Then out of pure love he is willing to sacrifice anything for her. It is this love that prevents him from breaking his vow to love and be constantly united with his wife. This "mystery" must be about the special relationship that God has with us, exemplified by the relationship between a man and his wife. That is why marriage is a holy union. The ideal marital relationship is one that is described in the above passage of Ephesians.