Ok, so Bristol is 18 years old and has a baby. And apparently she's the only one.

She was in as committed relationship as a 17 year old can be in, she accepted the consequences of her actions, and she is using her public status as a way to possibly educate other teens on the seriousness of those actions. Some people may not understand what she meant when she said "If girls realized the consequences of sex, nobody would be having sex" (even when it's explained

) and that she's being an advocate for teen-pregnancy prevention. That doesn't mean that she's an idiot, or that she would get "knocked up" by any guy who paid attention to her.
Would it be OK for the joke to have been about a black woman on welfare with 6 kids by 6 different men? Oh no! The NAACP and ACLU would have been all over that one. But a prominent Republican's daughter gets pregnant while still in high school,
finishes school, still plans to go to college, works 2 part-time jobs to at least help her family with the expenses, and gives interviews to prominent magazines and television shows to advocate teen-pregnancy prevention (which is not abstenence only education) is 100% open to ridicule.
Sarah Palin may have made the argument political. Honestly I didn't see that part of her statements against Letterman, but it doesn't surprise me. That, IMO, does not negate the fact that it is seen as acceptable to assume that if a woman has sex with one man outside of marriage, she would automatically be a slut. Why is that OK? How is that defensible?