I'm a Christian who sees a clear divide between moral and civil law. I believe that God created the institute of marriage, and that He intended marriage for one man and one woman. I believe that homosexual behaviour (not attraction) is wrong, as defined by God's moral law.
What God says is right and wrong is perfect, because He is perfect. Any civil law that we create is never going to be perfect: in a democracy, in fact, it is rarely anything other than popular consensus. I therefore do not care what civil law says about marriage, and would not expend energy opposing any further changes to our laws.
Any person who does not believe in God, or rejects Him, is inevitably going to regard marriage as a civil institute only - and in a sense, this is true, as determined by the participants' attitudes, just as if I were to marry, it would be primarily a religious rather than civil act because I chose that it be so.
If I had untold power, then I could hypothetically force people to comply with the definition of marriage as between one man and one woman only. This would also be futile, because it would do absolutely nothing to solve anyone's deepest need. I believe said need to be redemption by and relationship with God; and this is something that nobody can be forced into. As John Milton suggests, 'God left free the will', and even He himself will not force anybody to come to relationship with Him if they do not want to.