Somebody was (reportedly) rude. This is only news when the person being rude is male and the person on the receiving end is female. THEN it's proof of how horrible men are.
It's really funny that you would pick an XKCD that is about how sexism works as part of your argument that it wasn't sexism, merely rudeness.
The whole point is that a man being rude about another man in this particular way -- by superfluously mentioning his attractiveness -- is exceedingly rare, while for women it is all too common. One would be an aberration, the other is pattern, and the pattern is why it's at the top of HN.
ugh. Sexism is an attitude, the rude introduction was the action. Who cares why it was rude? It shouldn't be more offensive because you can (correctly or not) ascribe a motive to it that you find distasteful.
Trying to decry this sort of thing and speak up in "defense of women" or any of that garbage infantilizes women and perpetuates a much more serious problem than men noticing how we look.
It's not only then! It also works, for example, when the person being rude is white and the person on the receiving end is black. I'm sure you'll figure out the pattern.
Kind of like the reverse of: http://xkcd.com/385