男女 is used only for human being. 公母 is normally used on animals. 雄雌 is more academical.

公母 is usually used with terms closer to everyday life: 公鸡/母鸡 cock/hen

In biological terminologies, usually 雄雌 would be used: 雄蕊/雌蕊 [stamen][1]/[carpel][2]


  [1]: http://en.wikipedia.org/wiki/Stamen
  [2]: http://en.wikipedia.org/wiki/Gynoecium