I'll give it a shot, though I can't be certain without more context. I believe the author of this quote is trying to say that, how we as a nation or community portray women is a reflection or representation of our character and predispositions. Media industry reacts to the desires of the public and shows women accordingly. The media will portray women as the community wishes them to be portrayed, and you can tell about they type of people that belong to a nation by the end result. To try and put it even more simply - If you look at how women are treated in the media, that will let you know what that nation's citizens are like.

It means " ... those who define the public agenda, and therefore the media agenda." I agree with you that the sentence is written in a confusing way. There should be a comma after "media" in the original. "public agenda" means what the public (people collectively) are interested in, what they want to see, etc. The author believes that there are "those" who define these things. Perhaps the author tells us who these people are somewhere in the article. Hope this helps.