Calling the genders in plural 'masculine' and 'feminine' is oversimplifying. Their names could be rather translated to "masculine personal" and "not masculine-personal". Therefore the only* nouns that are "really" masculine in plural, are groups of men-only and groups of mixed gender. Groups of women, and all the other words, are 'feminine', or rather 'not masculine-personal'.
*Okay, that's another simplification. It also works for personified animals of male gender (like in cartoons) and for something else which I cannot remember nor find right now.