To get into it a little more, think of the Kinsey scale. The two extremes on the end are 100% heterosexuality and 100% homosexuality, then everything in between can be described as bisexual. Even people who are 90% one side and 10% the other. When you put it that way, a majority of the world would be considered bisexual and a minority exclusively hetero or homo.
So maybe most of the world is bisexual in some way, sure...but not everybody.