Is Sarah Palin’s comment sexist? Yes.
Why does she think this is acceptable? Because Sarah Palin says sexist things, and buys into sexist ideologies and frameworks. Why wouldn’t she think it was acceptable?
Are telling a man to grow a big stick, and saying a woman’s just on the rag comparable? No. “Grow a big stick” is an insult because it states that Obama is lacking in both masculinity and male genitalia; he isn’t man enough. “She’s just on the rag” is dismissive because it states that the woman in question has an overabundance of femininity and female reproductive functions; she’s too woman. They are the exact opposite in terms of why they are insulting. They do both, however, agree that Being A Man is good and Being A Woman is bad.