hey. we have a women + trans night during the summer months. i'd second what others said, but emphasize to tell people that women and trans nights happen all over the continent, and that they happen IN RESPONSE to unbalanced access to bike shops, not to increase it. you can mention that it lets more people use the space than would otherwise.
i think that is the right way to look at things, but people already opposed to women's nights may not, so...
another thing you could consider if you are not already open 7 days a week: tell the complainers if they want more shop time, they can always become volunteers to try help keep the shop open more days for everyone, rather than trying to limit women & transfolk's access. this could always backfire, i suppose, if they became volunteers and then tried to argue to eliminate women's night inside the organization...
macho