You all know it, so I should say it. This..
More seriously he invites attacks from people who think his notion that Palin is "the first indisputably fertile female to dare to dance with the big dogs" is fairly ridiculous. Indisputable to who? The underlying argument holds that anyone whom the author doesn't deem attractive, is somehow disputably fertile.
...is an erroneous reading of Purdum. I stand by the broader conclusion, but that particular claim, on reflection, doesn't hold up. More likely, Purdum was (as many commenters have pointed out) simply alluding to Trig's recent birth.