First – the fact that this guy's employer made a kneejerk reaction is no one's fault but the employer.
Next – let's talk about this red herring of "sexism." Making a phallus joke might be sexist depending on context. Or it might not be. Sexism is not at issue here.
The individual in question concedes that they did, in fact, make the joke.
From PyCon's code of conduct:
"All communication should be appropriate for a professional audience including people of many different backgrounds. Sexual language and imagery is not appropriate for any conference venue, including talks."
A joke about dicks? That seems to run afoul of the code of conduct the attendees agreed to. Sexist or not.
There's a discussion to be had around the best way to confront these behaviors. But regardless of your position on that, it remains the employer's decision, and therefore their mistake, to fire this person.
(Disclosure: I am acquainted with Adria and we've appeared together on a panel.)