Disappointing, but not surprising, really, when one considers that back in 1991 Dowd happily penned for the Times' front page a "news" article hyping Kitty Kelley's scurrilous (still unproven) accusations against Nancy Reagan. That's just the way she rolls -- which tells you more about Ms. Dowd than about the First Ladies at issue.
As for fictionalizing the First Ladies' sex lives, well, that's been done, too (actually, in fact, probably by Kitty Kelley herself -- see above -- although her books were supposedly "true."). I remember reading a spectacularly bad novel (reviewed, incidentally, in The Times) in the summer of 1990. The two main characters were thinly (very thinly) fictionalized versions of Nancy Reagan and Joan Kennedy. The Kennedy character was (of course!) supposed to the the heroine (despite drinking, affairs and assorted scuzziness), and you can imagine how the Reagan character was portrayed. The reason I even remember this novel at all was because I was so appalled that some hack would try to insert herself into the First Families' bedrooms.
But again, that kind of unimaginative garbage just tells you more about the people writing it (and buying and reading it) than about the person it purports to embarrass. It's a pretty sad sign of the times when this kind of crass behavior isn't treated with the contempt it deserves -- but, sadly, is anyone really surprised?