There are many views of marriage today. Some have felt compelled to fight for its “true” meaning. The odds of victory on that seem very slim.
Adding the adjective “Christian” shifts the struggle from defining “marriage” to defining “Christian.” There will still be differences of opinion but at least you have some standards to work with: the Bible, history and tradition. This is the same way that a Muslim or a Hindu would approach their definition of marriage.
I’m glad that we have the option of a secular wedding at the court house. Calling that a “civil union” might be more accurate, as it is essentially a business contract, though some folks feel “civil union” is somehow second-class. Fine. If you want to call it a “marriage”, why not? You might say a “legal marriage” or “non-religious marriage” or “state marriage”. Doesn’t really matter. But its not a “Christian” marriage.
For most of church history Christian marriage has been viewed as a sacrament. Like Baptism, it is something that is given or done by the church and received by the couple, a sort of gift as well as a covenant. It is proper for the couple to speak of their marriage in the passive voice: “We were married by Fr. Jones.” And for Fr. Jones to speak of it in the active voice: “I married Bob and Sally.” Swap “baptized” for “married” and you get the idea.
Not everyone wants to receive the gift of sacramental Christian marriage. It should not be forced on anyone. It is a totally different reality than the courthouse contract. If you call the civil union a “marriage” you definitely need an adjective (at least) to describe a Christian union.
This video does a great job of illustrating C.S. Lewis’ clear teaching of how most Christians have understood marriage until pretty recently, and how we at St. Peter’s still do.