This thing is starting to bother me and as my initial reaction was far from complete, here's the final word. Andrew Sullivan is absolutely right when he argues that:

Candidates mention their families all the time. An entire question last night was devoted to the relationship between men and their wives and daughters. Mentioning Mary Cheney is no more and no less offensive than that.

Yes, we should be free to talk about Mary Cheney and her lesbianism because it’s perfectly normal and accepted, equating it with alcoholism (one of Andrew's readers offered up Betty Ford) is perilously close to equating homosexuality to disease. So Andrew’s train of thought works fine, but it stops when someone else ventures into family territory unasked, like Senator Kerry did last night. Yes, Mary Cheney is a public person but Kerry did not talk about Mary as such, he: (a) brought her in the discussion as the daughter of Dick Cheney, the Vice President and (b) made an assumption about how she felt about herself. In fact, Mary Cheney the public person was never discussed at all:

We're all God's children, Bob. And I think if you were to talk to Dick Cheney's daughter, who is a lesbian, she would tell you that she's being who she was, she's being who she was born as.

No parent can empathize with a child being brought into a public debate in this manner which is why the Cheneys are rightly pissed off, although Dick and Lynne are shrewd enough to use this fracas to score a few political points.

UPDATE: Power Line makes a similar argument and has a revealing anecdote about Mary.