So? It's not hurting anyone. You know, there is a difference between an ad for a geek event that no one cares about and a woman not getting a job because she's a woman. These are not the same things.
I can imagine many women feeling unwelcome at an event that was advertised in that way. I don't want women to feel unwelcome at tech events for many reasons.
I don't see any evidence that they were just trying to be humorous. "Women" is listed right after "booze". If they were joking by advertising "women" as "perks", then they should have understood that most people are not going to find that particularly funny.
They advertised "friendly female event staff to get you a beer". Any reasonable person would believe that they were intending to follow through on that.
Well, did you expect to see a "WARNING! WE NOW WILL ATTEMPT A POTENTIALLY OFFESIVE JOKE" written in red print? Those things are supposed to be spontaneous, you know.