It's interesting that Harper doesn't want to touch social issues, since he was pro-life and traditional marriage. Forgive me for stating the obvious, but I believe that Harper's reluctance to deal with such issues is part of a greater goal of replacing the Grits with the Tories as the de facto ruling party of Canada.
Regardless of other political reasons involved, one would have to assume that Harper is very wary of reopening the marriage debate; any effort to do so on his part would inevitably push into the limelight some elements of his marriage he certainly prefers to keep away from public discourse...