If we preach the sanctity of marriage then let every man live with the wife of his youth (Mal. 2) and let our witness to the world be one of honoring that institution as holy. If we are indistinguishable from the world in our divorce rates, our adultery, our fornications, then how can we proclaim that political recognition of homosexual marriage threatens "family values"?
The overall post is an excellent argument for Christians to watch the way they describe their views and to serve as a model for reasoned discourse. The reason most people think evangelical Christians are stupid and have idiotic views is because enough of the people who claim to speak for evangelicals on political issues make themselves look stupid and make their views look idiotic, even if they're not. But what's just as important as learning how to speak into the mindset of a culture that shares very little of your underlying convictions, that finds very little of your assumptions plausible, is whether you live what you say.