For Christians, the basic notion of marriage involves God. It is theistic from the get-go. Primarily, it is about God and the two individuals being married. This is why priests officiate.
Yet, public and civic discourse in the West is increasingly secular – a basic premise is operational atheism. Marriage is not about God, and this is why judges increasingly are the ones who officiate.
This is why much of the public marriage debate isn’t a debate about things, but about words. Until Christians are able to recognize what has happened, their arguments won’t make any sense.
To put it simply, to regain a Christian understanding of marriage in the public sphere would require making civic discourse theistic in its basic premises. The changes in marriage are downstream of this.
What can Christians do about this? First, simply realize that civic discourse is no longer Christian, and that this is what has caused the dislocation of Christian marriage. Second, insist in discussion about marriage that what they’re talking about by marriage is something that is first and foremost about God. From that comes conceptual clarity.