This is always where geeks run off the rails in discussions like this. We want to come up with rules for things: either this statement is "sexist" or it is not, and here is the objective reason for that decision.
But it misses the point. Context matters. In a utopian environment where women felt equally respected, sure, a sideways/slightly-inappropriate comment would be fine. In some workplaces today that might even be true. But it's not the norm, and in the real world people feel disrespected by this kind of treatment. Not everyone (but some, sure, as I'm sure someone will point out needlessly) likes the fact that their perfectly "normal" clothing choice can make them "attractive" in one context but the butt of a joke in another. Men, on the whole, don't deal with that.
But it misses the point. Context matters. In a utopian environment where women felt equally respected, sure, a sideways/slightly-inappropriate comment would be fine. In some workplaces today that might even be true. But it's not the norm, and in the real world people feel disrespected by this kind of treatment. Not everyone (but some, sure, as I'm sure someone will point out needlessly) likes the fact that their perfectly "normal" clothing choice can make them "attractive" in one context but the butt of a joke in another. Men, on the whole, don't deal with that.