And of course we should talk about polygyny, not polygamy. Polygamy just menas that one person has many spouses, but polygyny means that one man has many wives. Of course we are always talking about one man haveing many wives, never about one woman having many husbands. There is in fact a word for the latter phenomenon, polyandry, but that sort of thing hardly exists in the real world.
While it is uncommon, it's not as if it doesn't exist in the "real world", as you put it. There are cultures in Siberia and Nepal that I know for sure practice it, as well as Tibet (though it's not as common in Tibert as it used to be, if at all). Fraternal polyandry is what I've heard the most about, and actually is very practical, given some aspects of their culture.
