@Mr Gharial I believe they do have, or at least at some point had, a male, but I believe he is (or was) usually kept in one of the exhibits in the Safari Resort (namely the one you can partially see from the car safari when it goes through the giraffe plains). I saw a male Nile lechwe in that area during previous visits.
The size of Beekse Bergen has the advantage that they have several different exhibits to put male or surplus animals in.
I do agree they should also have depicted the females.
