First of all, polygamy only works in a theistic culture where marriage is required for sexual relations. Otherwise, you'd just commit adultery.
So, let's say a guy falls in love and gets married. Then, one day, he sees another girl and is attracted to her.
But, how can this be? He's married. UNLESS, God wants him to be with that girl, too. After all, he's "naturally" attracted to her. Only problem is, he still loves his first wife, and doesn't want to get divorced either.
Wait a minute....I think I have an idea.
And there you have it.
But, what if your wife has similar "natural" feelings toward another man? Well, that's the devil, of course.