Since sexism is a function of (shared) subjectivity, the comment could be believed, but it wouldn't acquire a degree of truth beyond that.

The OP said that 'there are a lot of women in SF who are rather plain looking but due to gender imbalance act as if they were extremely attractive'.

To me that this statement can be true or false (I've never been to SF so have no idea). It either is or it's not true.

Separately from that, one can argue 'It's sexist to grade women based on their looks and to likewise imply that attractive women can be expected to act differently than homely ones'.

"Plain looking" can be true or false? By what standard? It's also an opinion that the way a person acts is due to demographics. tl;dr: neither true nor false, just a belief, a value judgement.

Some people in this world would definitely say that it's not sexist to grade women on appearance, and in fact it is a value judgement held in positive regard in many parts of the world. That there is a class distinction that implies standards of behavior follows from this.

Right - it's a value judgement, I agree. With which one can agree or disagree (ie find it true or false - not in a axiomatic sense that 2+2=4 true, but in a 'most people find it so' true).

But the commenter who said 'it's sexist, nothing to discuss' avoids have to make that judgement. He prefers to just say 'I choose to label certain topics taboo (ie label sexist), hence avoid having to discuss them'

You may want to read the entire paragraph - he concludes with this phrase - "doesn’t give you the right to be a bitch all the time". He's basically saying that he thinks attractive girls (9s) behave like bitches all the time.

This isn't about value judgements of attractiveness, nasty enough as that can be.