God’s plan from the beginning was to have perfect fellowship and freedom with His creation.  Perfect fellowship is perfect unity.  God determined that man (Adam) needed a suitable helper (Eve) that could complete him (Adam).  The physical, emotional and spiritual unity that comes through marriage was God’s plan for instructing mankind in the ways of unity while preserving his freedom.  This teaching tool of His achieves the “greatest good” by increasing the number of free moral agents who can choose to love God and find this perfect fellowship and freedom in Him.

 Why God chose the vehicles of male and female for this are ultimately known by Him alone; but I think that we can see why marriage between a man and woman achieves God’s aim while no other union can.  One likely reason for the biological equipment that men and women have is that they bring great pleasure and productivity.  Remember, sex is a part of marriage and marriage is about unity.  Unity with God is a form of ecstasy that cannot be compared to any physical or temporal experience; however, God will use those kinds of experiences to help “conform us to the image of His Son” as He teaches us how fantastic unity is.  This is all a set up for eternity.  

God has made the union of a man and woman a sacred and holy thing; this is why the union that isn’t between a married man and a woman is disgusting to God.  Sexual unions that are not part of the marriage relationship between a man and a woman “de-sacralize” what God has made sacred.  Fornication, adultery and homosexuality reject the design of God in favor of selfishness.  They are perversion of freedom and unity, the things that God is most concerned with in our lives.