Perhaps the "beer" subtext is just more about being exclusive (beer nerds welcome here) versus inclusive. What if instead the same event was going out to a bar, where you can order a whiskey, cosmopolitan or soda?

I think it fits into the greater "culture" of a workplace, and is at most a symptom of the problem and not the answer. If a CTO thinks his ads are appearing "sexist," the problem must go deeper than the keg in the break room (which is just the most obvious sign of immaturity.)