I think the angry reaction to Kerry's mention of Mary Cheney points out some gaps between People Like Us (youngish, educated, urbane, politically attentive) and the rest of the nation. (I consider myself one of People Like Us -- and I've worked hard to make that happen!)
1. People Like Us knew that Mary Cheney is 35 years old, partnered, utterly out as a lesbian, and active in the Bush campaign. Many of us were wondering how the GOP was going to handle her, particularly in a year when same-sex marriage is a prominent issue and many Republicans are conducting gay-baiting campaigns (e.g., Jim DeMint, Alan Keyes, Tom Coburn, the infamous "Ban the Bible" flyer in West Virginia). Probably most casual viewers didn't know about Mary. Probably many of them imagined her as an 18-year-old "experimenting" deep in the closet.
2. People Like Us see homosexuality as no big deal. Saying Mary Cheney is a lesbian is like saying Alexandra Kerry is a medical student or that Jenna Bush graduated from the University of Texas -- maybe irrelevant or uninteresting, but not slander. Many older, more socially conservative people see it as equivalent to asking Kerry about his divorce or Bush about his alcoholism -- something far too personal to be talked about on national TV. Others see it more like discussing a family member's abortion or drug use -- something they may be ashamed of and wish to keep secret. Andrew Sullivan talks about this here.
That said, Kerry's statement was still weird and inappropriate, although I am less convinced that it was part of a deliberate political strategy. ( I also think the interchange between Lynne Cheney and Elizabeth Edwards came much too close to an argument over sound parenting techniques). I wonder whether the Cheney family's reaction -- so different from their response to Edwards's discussion of Mary -- had been planned in advance -- Lynne will scream her head off, Dick will react in a more restrained manner, Liz will demand an apology.