There are some things people in these threads are chronically getting wrong.
First, lots of people are making the claim that this was a "private" joke. It was not. If it was a private joke and they shouldn't be punished for that reason, then they should have been able to say anything, and as long as everyone who was meant to be in the private conversation appreciated it, it would be wrong to punish them because it was private.
So here's an example: What if they made a joke wherein they said, "Let's lynch a "? They likely would have been kicked out, could easily have been fired, and no one would have much sympathy for them. Why? Because it's not in private. They were at a conference, which is a community gathering with community standards to make sure that everyone feels welcome, and they were talking when literally surrounded by attendees they don't know. What they say absolutely has an effect on how welcome others feel. We've decided that it's important enough that all feel welcome that we've made codes of conduct.
So once we've established that there are things they could have said that would make their behavior unacceptable, you're now saying "that comment didn't warrant this response", not "this was a private conversation! how could they ever kick them out or fire them for that?"
And at that point, we get to all the arguments that this joke "wasn't sexist". It is true, that the joke itself was not sexist; it did not objectify women, suggest that they are less than men, etc.
But it was contributing to aspects of the tech industry's culture that make it less welcoming to women, and for that reason, it is unacceptable at a professional conference. (It's also just crude and might make some men uncomfortable, and so is unacceptable for that reason too.) In nearly every context that a woman in tech finds herself, she is surrounded by men. This can often make them feel somewhat unwelcome or uncomfortable. Even little things, like a few people who make comments suggesting that they're nontechnical, or a few people asking them out at every single meetup they go to, can make them feel less comfortable and less valued for what they came to the conference to do--be technical.
Anything that contributes to the feeling of the conference being a boy's club, or sexualizes the environment, can make this worse.
Finally, many people ask, "Why didn't she just say something to them?" While I don't think going straight to twitter was the best she could have done, I also don't think people understand how difficult it can be to approach someone and call them out on behavior that makes one feel uncomfortable, especially in a space where one already does not feel especially welcome or taken seriously. If everyone yells, "you should have just talked to them!" every time a woman immediately complains to organizers about inappropriate behavior, you're just encouraging more women to silently feel unwelcome, and likely drop out of the community, rather than speaking up and helping make the community better. I agree that, when comfortable, directly speaking with the offending party is best way to deal with these problems, but those who have been made uncomfortable have absolutely no responsibility to do so, and shouldn't be pushed to do it even if they are not comfortable with it.