ZippyMatch was built with scalability in mind. That said, one way to reduce information overload is to show you only the people you are interested in meeting. This is why we ask you for your gender, as well as your gender preferences when you register for an event.
That said, event organizers choose and name their own genders for an event. Organizers could literally choose "cops" and "robbers" as genders, and some organizers (especially those organizing LGBTQ events) simply use the phrase "Human" to describe people, and only provide one gender option. It all depends on the scope of the event, if there's any theming to consider, among other considerations.
We understand this can be a sensitive issue. If you are unhappy with the genders used for a particular event, you can contact the organizer of the event to discuss it.