I think that they're right as far as what Christianity says about marriage. However, think they miss the point some of the rest of us have about the law. I just don't find anything in the Bible that suggests that Christians are supposed to impose canons on people who are not Christians. There's no command to impose the Bible, it's only binding on Christians, it's not like Shariah where nonmuslims must be bound by Muslim law.
America isn't a Christian nation, from the very beginning it was founded on the Englightenment, not Anglican law or Catholic law or Baptist law.