I agree it was a very sexist thing to write. It makes me wonder though:
1. Which is worse, hiring an all female serving staff (perhaps wearing uniforms that would be considered particularly attractive to heterosexual males), or acknowledging that you have done this by writing about it in your invitations? Is it inappropriate (always and everywhere) to hire only attractive females and make them wear "sexy" uniforms? If they had hired all females, but not advertised it, would people still be upset after they had arrived and seen that all the staff were female? I wouldn't be upset. But advertising it, or at least they way they did it, strikes me as offensive, but I don't know why.
2. I feel mentioning the female serving staff in this instance is inappropriate, but is it always inappropriate? Is it acceptable for a sports bar to mention their "Bud Girls" or whatever in their ad copy? I would consider this acceptable, but I don't know why. Is it because a sports bar is purely leisure, and a hackathon, while leisure, is (presumably) directly connected to your profession?
I think what they did feels wrong, but I can't think of a good explanation of why it is wrong that is logically consistent with other acceptable norms in our society.