Maybe this can help explain. My recent ex-girlfriend is female, but is also bisexual and so likes women almost as much as I do. She was perfectly happy to comment on the attractiveness of the waitstaff to me (male or female), and appreciated when I did likewise to her. She'd be perfectly fine with a social event advertising attractive female waiters. I think she might even have been happy about the prospect of attractive female waitstaff at a hackathon too (she's also a programmer).

Listing the staff under the category of "Women" though, with the implication that these were going to be the only women at the event. And that would really piss her off.