I don't particularly trust the source, but when I went looking for recent data, this is what came up:
In the new survey, the only age group in which a majority opposes same-sex marriage are those 65 and older, and the only region with majority opposition is the South.
That supports what Will said. Where is the evidence to the contrary?
By asking for the “evidence to the contrary”, you imply that you do trust the source.
And if this were a one-time or one-issue thing out of Mr. Will, it may be considered an anomaly. But it is not.