"Sexism" is prejudice or stereotyping against someone on the basis of their gender. Simply calling someone a bad word or insulting them is not sexist.
Barack Obama didn't say anything about Gov. Palin in the first place, and if he had, it was not sexist. But the important thing is that he wasn't talking about Sarah Palin.
The part of The Rabbi's post that I appreciate most is the inclusion of this news report from Keith Olbermann, at MSNBC:
"Apparently a rape joke is authentic McCain!" Well said, Mr. Olberman.
See the post at The Rabbi Report here.