Marriage Equality And Religious Freedom

A reader makes a point that isn't made often enough:

I am another gay man who has no problem with a church refusing to conduct a same-gender marriage rite.  What I don't understand is why the conservatives/fundamentalists can't get it through their collective skull that their insistence upon enforcing in civil law their particular interpretation of theology is also an excercise in religious discrimination. 

The Unitarians have been marrying same-sex couples for some thirty years, and likewise some congregations of the United Church of Christ, the Metropolitan Community Church, and I'm sure a number of other religious groups I don't even know. Why do the fundamentalists get to discriminate with the force of civil law against the U/U, the UCC, and the rest? When did they get the right to have their religious interpretation enshrined in civil law at the unavoidably explicit expense of the others ' interpretation?

When did they get the right to be the government's de facto Department of Inquisition?

This struggle is not just between secularists and Christianists. It's also between Christians.