"'For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh.'" NIV translation
Paul now quotes from Genesis chapter two and we are reminded why God created men and women. Adam had named all of the creatures on the earth and they all had helpers (mates) but there was not a suitable helper for him. That is why God created Eve from the very flesh and bone of Adam and the two were meant to be together. The simple fact of the matter is that God created a man and a woman for each other and neither one will ever be whole without the other in their life. Many times we as humans just settle for any mate that comes along but we must remember that God has created one just for us and He knows what He is doing.
"This is a profound mystery - but I am talking about Christ and the church." NIV translation
One of the mysteries that Paul is talking about is the fact that Jesus left His Father in heaven to come down here for us. He lived a perfect life here and then gave that life for us (His bride). We are joined to Him through our faith and the Father sees us as one.
Another part of this mystery for Paul would have been the fact that Jesus did this for Gentiles (non-Jews) as well as for Jews. We all must be joined as a part of His bride that is the church.
"However, each one of you also must love his wife as he loves himself, and the wife must respect her husband." NIV translation
Just as believers are made one with Christ and walk in His light, our walking in the light begins at home and loving our wives. This is a sacrificial love just as Jesus gave up His humanly life for the church. We, as husbands are called to regard the needs of our spouse as if they were our own because we are one. If husbands do that, then the wives will not have any problem showing the respect that Paul says they must do. On our own, this would be impossible but we have the power to do this if we listen and obey the leading of God through the Holy Spirit.