Why do people think polygamy is bad?

Whatever floats your boat. But the women who practice this will never be truly happy. The man won't cherish her as he would if it was his only wife. Besides that, girls seem to be blind to the truly good men, so if a guy was aloud to have more than one then every smart/nice/good person would become extinct. Our whole society would collapse. Look at every truly great person in history and ask yourself "are they attractive?" The answer is usually no.