Letterman rape/pregnancy jokes about Palin girls

Apparently, David Letterman thinks it’s funny to describe Sarah Palin as ‘a slutty flight attendant’ and make jokes about her daughter getting ‘knocked up.’
‘The Alaskan governor was there for a baseball game and to be honoured by a special needs group, accompanied by Willow, 14.
“One awkward moment for Sarah Palin at the Yankee game,” Letterman then quipped on-air.
“During the seventh inning, her daughter was knocked up by Alex Rodriguez.”‘
Once more, from the top: it’s not okay to make rape jokes about someone just because they happen to be right-wing.
I’m British, and don’t particularly like Palin, or know Letterman at all, but this still pissed me off. His defense is that he was ‘talking about Bristol, not Willow.’ But even so, it’s still a pretty bad joke.