1- the ratio of men to women is not 50/50, as far as I know, in many countries there are more women then men ( http://en.wikipedia.org/wiki/List_of_countries_by_sex_ratio ). (note, you are also assuming that all of those men are *cough* straight!). Note that the table doesn't show you the population of each country, I am not sure what would the ratio be if you try to look at the world as a whole.
2- not _every_ man marry more then 1 wife, I lived in the middle east and now living in Africa, and in all those years I only knew 2 men in the middle east who had more then 1 wife and I only know 3 men in Africa who have more then 1 wife. The others, that I know, are only married to 1 wife (if married).