Their right to be treated as equal. How would you like it if a predominantly female group was told there would be men serving drinks? Predominantly racist group told there would be blacks serving drinks?
Some people might see it as a poor attempt at humor and not demeaning to women, but I'm sure others saw it as a reinforcement of their beliefs that women are for serving beer. When everyone laughs along, because they know nobody 'really means it' it sends these people the message that their views are OK and widely accepted.
I'm completely OK with a predominately female group being told men would be there serving drinks? African americans serving racists? That's a totally different ballgame.
OK, so how about blacks serving a predominantly white group? I think my other race metaphor went too far, but this one works. There's nothing wrong with a black person working as a server/waiter. They're still considered by most people to be completely equal, and blacks are welcome to participate in the event alongside whites, it just so happens that their drinks will be brought by blacks.
How does offering to have women bring you beer at an event take away your rights? Shall we be boycotting Oktoberfest next?