Quoting from Mere Christianity by C.S. Lewis on marriage (p86)
“My own view is that the churches should frankly recognise that the majority of the British People are not Christians, and, therefore, cannot be expected to live Christian lives. There ought to be two distinct types of marriage; one governed by the state with rules enforced by all citizens, the other governed by the Church with rules enforced by her by her own members. The distinction ought to be quite sharp, so that a man knows which couples are married in a Christian sense and which are not.”
The easiest solution to the problem of whether homosexuals are allowed to marry is to distinguish religious marriage from the civil contract “marriage”. People have a right to make contracts with each other on how to live together, how to make important life decisions together, how to be happy together etc.
A Christian Marriage is something different from the civil contract part of marriage. It is an arrangement where you become one person, one organism, that is for life. Jesus called it “becoming one flesh” between one man and one woman. It is not the role of the government to be the church, but it is the role of government to give equal protection under the law.