The problem, as with all sexist/racist/homophobic/etc jokes, is that even though you recognise that it's a joke, people who read it don't necessarily do so, and these kind of comments enable the people who believe this stuff to continue to say it, and to believe it's acceptable. When you're exposing your writing to potentially thousands of readers, you have to consider this.

It's how reddit went from joking ironically about sexism/racism/homophobia to now featuring pockets where that kind of thing thrives.

While I don't want to justify making these kind of jokes to your friends, it's very different when you're with five of your best mates who all recognise that you're joking and understand that that kind of comment is sexist, to saying that kind of thing on the internet.

This probably feels like an overreaction - I'm not offended by the comment, just trying to explain why having them is an issue.

Always wink it when it's a joke [0]

[0]: http://en.wikipedia.org/wiki/Poes_law

What about the sexism of Facebook's advertisement?

No. Hacker News is serious business.

No, because offending someone is the very worst thing you can do in the world.