As with some of the other arguments on this list, I can see two meanings for this one. The first related to "marriage" having religious connotations (e.g. "sanctity"), and the second related to the terms "marriages" and "unions" being interchangeable (i.e. secular). As for the former, I think it's rather clear that gay marriage proponents want to use this term in the law because it may hasten social acceptance. Nevertheless and notwithstanding what my personal position on the matter may be, I don't like the idea of forcing such a change by usurping religious concepts when a viable alternative is available, which brings me to the second possible meaning of this argument. If "union" is acceptable, then this argument loses most if not all of its strength since "marriages" would be defined by the church and "unions" by the state (though there would be a considerable overlap). For instance, a church could recognize a polygamous marriage but the state would not. To address some of the common claims in this thread, I'll add that many point to the currently high divorce rate, yet the "sanctity" proponents can simply counter that it's too easy to get a legal divorce by citing those statistics. In the final analysis, the strength of this argument depends on it's meaning, so I'm sitting on the fence.
